There is an old joke about the rhythm method of birth control, which is exactly what you are describing.
"There is a special name for people who practice the rhythm method... they're called parents."
It's been pretty much determined that the rhythm method is unreliable.
Sounds like you're trying to prevent a pregnancy. Keep in mind that every woman's cycle is different, most of us have our periods every 28 days; however, in my case, my cycle is every 29 days religiously. Please keep this in mind while reading the "answer" to your question. Early in your monthly cycle, several days AFTER our period ends, our estrogen (female hormone)levels rise which triggers the release of a cervical mucous (vaginal discharge). Throughout our cycle, this mucus increases in volume, changes from clear to a milky white as well as changes in texture. You ARE most fertile, when your mucus/vagional dischage is clear, slippery, and stretchy - this mucus resembles raw egg whites. In other words you are ovulating and most likely would get pregnant. You see it is this clear mucus/vaginal discharge that allows the sperm to enter the uterus and swim toward the fallopian tubes in search of an egg to fertilize. Our periods arrive to shed the unfertilized egg and uterine lining/blood. Periods allow our body to shed the lining of the uterus (what we see as blood/our period). Our body/uterus did not need the nutrients/blood that lined the uterus because no egg was fertilized therefore our body rids itself of the eggs "home" . After ovulation there is an increase in progesterone (another hormone) which makes our vagional mucus/vaginal discharge scant in supply, and the mucus changes from clear to a milky white appearance, the discharge is also thicker with a pasty feel. This type of mucus/vaginal discharge blocks the sperm's path through the cervix to the fallopian tubes. Knowing all of this information and if vigilant practiced the mucus method of monitoring the most fertile time of the month is only about 85% effective to prevent a pregnancy. There is only ONE method that is 100% effective; though, that method is abstinence--I implore you, if you are a teenager in the middle school or highschool to not have sex. 13 to 21 year old BOYS want 1 thing, the sex act, an orgasm, period! And, once you give in to their pressure I can state with 99.9% assurance, they will not call you again nor do they respect you any longer-if they do call after sex, it will be because your one of the girls who will put out! Boys like, respect and will continue to date the girl that doesn't put out. And, your wording of your question says a lot ifyou think about it; you ask what are the safe days to have sex. Darling, it's not a basketball game or a wrestling match, it is making love! Do not give that part of yourself away freely or easily ever! If you do you will not respect yourself nor will you be respected-boys don't want to be the girls that "put out" I am 35, I was a virgin until I was 20, I lost my virginity to a man that I had a 2 yr relationship with, he never pushed or pressured me either because he respected and love ME. Keep that as special, price it high and don't sell yourself out to be cheap! Now, if you by chance are a full grown woman, ect., disregard the last few sentences of this email! :) If you're not, take what I'm saying to heart! Boys are selfish, they do not become a man until they reach the age of 25 +/- and, even at age 25, trust me, they're still growing up! The only difference between men and boys is the price of their toys---and that is the truth, God love 'em! Good luck!
IMPROVEMENT/EDIT: The only known method of having 100% safe sex is condoms + proper hygiene + backup method, WITH A PERSON YOU TRUST. It consists of properly using a well-fitted condom properly. That includes, ALWAYS, the guy paying attention to if it suddenly starts feeling 'different' (I've had sex maybe 1000 times, broken exactly 4 condoms, felt it each time and safely withdrew and replaced it - if I were concerned about STDs with those partners, I'd also have used antiseptic antibacterial soap and scoured my genital area with it the moment I realized there had been accidental genital-genital contact, but that was not a concern, condoms were used for pregnancy prevention purposes only in these cases), discreetly checking the condom during long or hard intercourse by fully pulling out while looking down (can be made part of the sex play, easily). It also includes immediately pulling out after ejaculation, holding the condom to the penis to make sure it doesn't slip off, then turning away, removing the condom without spilling the contents, and ALWAYS TYING IT OFF (!). Why? That seals it and creates an air bubble, with pressure, inside - like a balloon - so if it had a leak, you'll see it.
Why a condom always? A) you know it's there, it's visible (pill can be forgotten, or lied about), B) it protects against both pregnancy AND disease (remember, people lie, people betray, people break trust... no amount of clean bills of health and promises will ever protect you if a partner is carelessly unfaithful... moreso, the more domestic the partner, the higher chances that if he/she ever cheats, it'll be with an extremely unsafe person like a prostitute/stripper/giggolo/fratboy/etc.... in fact, one-man-for-life faithful married or dating-engaged to their first guy women, due to blind trust of reciprocal feeling, are actually *more* likely to get STDs than a promiscuous but careful woman), C) with condoms, if you use em properly and TIE THEM OFF, you'll know if they failed and sprung a leak; if they slip off you feel it, and if they broke into pieces, you'll likely feel it early too (though don't bet on it if you're both inexperienced, hormone-crazed teens, the novelty and intensity of it makes you unlikely to ever notice anything - whether a breaking/falling off condom, or your entire extended family walking in on you and staring in shocked disapproval... first experiences with sex are total tunnel vision, nuclear bombs could go off next door and you might not even notice).
In case of a condom leak, backup methods kick in. Those are: antiseptic sprays/soaps/etc. for disease protection, some other contraceptive method for pregnancy prevention. The secondary contraceptive method, ideally, if you suffer no side effects, would already be in effect - a long-term contraceptive shot, an hormonal implant, or hormonal pills ("the pill") of the daily variety. If not, apply a spermicide, hell, suffer the potential irritation or whatever and apply as much of it as you've got, the full package not one dose. You should have one of those on hand, already. Otherwise, go for plan B (aka emergency contraceptive pill, also can use 5 doses taken at once of SOME daily prescription contraceptive as that is what Plan B is, it's five doses in one of one of the hormonal contraception tablets colloquially known as "the pill" - know in advance whether you have the right one and have quick access to it always, if pharmacies are closed on weekends and holidays or are too far away or something - own some, in advance). If you don't find it morally controversial and are more afraid of preganancy than the unwellness and discomfort that plan B causes (it's an off-schedule instant period-causing pill, essentially), do that AND the spermicide immediately (unless already on birth control pill, then disregard plan B). Do NOT shower the vagina if there was a spill inside and you're more concerned about pregnancy than disease: if not using Plan B or the pill, it could drive the sperm up to the egg and cause pregnancy. There are antiseptic sprays reputed to kill some diseases, to be used along with the spermicide if that's your backup choice, but their reliability is suspect at best, and sometimes utterly disputed.
The hygiene part: the male, after ejaculating, immediately removes and ties off the condom. If everything is well and the condom is unbroken, he does not get to touch the girl, with any part of himself, AT ALL, before heading to the bathroom and washing his hands and penis and genital area in hot water and soap (obviously if neither of your mouths were anywhere near anyone's genitals, a handsfree kiss to the face or lips is harmless, but really, make sure nothing that touched the condom or your penis, or that touched any part of anyone that touched the condom or the penis, goes on to touch the girl, anywhere!). Classic unlikely but common cause of pregnancy with condoms is: guy uses condom, removes it, goes on to manually stimulate girl with sperm on his fingers/ girl goes down on guy to orally stimulate him back up for another round, then pulls up gives him a kiss, and he decides to return the oral favour - and brings his own sperm, via the girl's lips to his own, right back "on target". OOOPS. So yes, it's like hazmat rules. If you've touched it, you don't touch ANYTHING before you decontaminate any areas you touched it with (normally the guy's hands and his genital area). Decontamination being, again, soap and hot water...
TRUST: that last part. If inexperienced or not sure that you're 100% in the lead and the dominant side in the relationship, never ever EVER enter into sexual relations with someone you don't trust. Guys have been known to remove condoms or claim to wear one in the dark (typical), jealous or desperate or greedy persons of both genders regularly sabotage birth control (punch holes in condoms, switch out pills, claim to use the pill when they're not, etc.) to cause pregnancy in the hopes of using the resulting child to force the other person to marry to hold on to them, get their money, get green cards, etc.; , people with STDs have been known to sabotage condoms or otherwise contaminate their partner on purpose so that "they're the same, now" for shame reasons, or to force a partner to limit their choice to the infected (thus: the very person that gave them the STD), etc etc etc. Yes, if you're a bigger guy with a smaller girl, his own condoms, controls of their usage and disposal, and prudent safety measures is generally safer from sabotage and other foul play, and is way less likely to be threatened into something against his will - but even then, trust is essential, just in case.... And if you're a physically weak or average young virgin girl, make bloody damn sure your first is the most trustworthy and loving bloke you've ever met!
Without trust, nothing is ever safe, especially sex. At later ages, you might grow into relationships where you wouldn't trust your partner if he were asked to answer the phone for 5 minutes, much less do anything responsible, but know 100% that the person is harmless and will never hurt or wrong you and trust them to agree to your judgment calls - such cases and what to do with them are up to you if you're an experienced adult and you can make sure that partner never has anything to "take responsibility" for, since you know they'll fail miserably.... But that's for when you're in your mid-twenties or later, when you're experienced, when you know what you're doing.
If you're a VIRGIN, you MUST trust your partner IMPLICITLY. In everything. Especially if that person, too, is inexperienced...
There is NO safe sex days, safe sex pills, safe sex shots, safe sex cervical caps, etc etc, not for virgins. They don't exist, period. You don't know enough to make an informed decision to take such risks. You might be blinded by new feelings (most cases, even the worst womanizers or party girls had a first they figured would be their one, their only, their last). Yes, even that chick in porn, and that celebrity who told the enquirer he slept with 15 thousand women... First times and first loves can be magical, too magical, you get swept up in the moment and you think you can do no wrong and have no wrong done unto you. Or make stupid decisions, like promise to elope to Vegas the next morning and decide it's hardly a bad thing if you accidentally conceive your firstborn the night before... but, come morning, reality kicks in. Either that, or you pay the divorce lawyers quite a bit 3 months later, or, better yet, waste half your lives unhappy, and then spend your entire life savings and everything you own on divorce lawyers trying to correct the mistake 30 years down the line.
So, double up your contraception, condoms as first line, other methods for backup.
PS double up does NOT mean wearing two condoms. that actually makes it almost GUARANTEED TO BREAK, never wear two condoms at a time!
PPS ignore the first poster about all guys wanting hit-and-run sex in their teens, and all girls falling victim to it. It's simply not true. If you've been dating for a while, and are friends first, makeout buddies second, it's almost guaranteed to not be true... and if you're both the same age (girls mature sexually, on average, 2 years earlier than guys), or especially if the boy is younger, chances are it's hoping for "forever" or at least and indefinite long term, and insecure or even terrified, fearing you're just playing or will leave for an older, more confident and experienced bloke... and yeah, guys too get played, get dumped, get hurt. The only reason girls are pigeonholed into the "victim-to-be" mindset is because, if people screw up and kids happen, you're the ones stuck with them - that's why proper knowledge of contraception evens the stage and is essential. Well, there is too the ugly specter of sexual violence, from which people just try to protect you, but honestly, I've known too many females of various ages that have been victims of it, and comparing nasty experiences - not to demean or belittle anyone - but it's just another form of extremely nasty violence. Yeah, girls are at higher risk of sexual violence, but guys are at higher risk of getting shot, clubbed down with a baseball bat, cut to ribbons with a knife, etc. - and ultimately, it's the same people doing these things, for the same angry reasons. It's not gender-based temporary near-criminal insanity, like overbearing relatives might claim.... Simply put: Boys don't rape girls; it's CRIMINALS that do violence unto normal people. Different shape, same sorta damage... An ethics-minded upstanding pacifist, a caring person genuinely distressed by violence, no matter dominant a personality, how many times bigger he is than you, or how many times he masturbates daily or daydreams about your body, will never try to rape you. Meanwhile a shorter than you, weaker than you, younger than you "follower" who barely looks like he's hit puberty, but has spent years beating kids up for their lunch money or relishes attacking his enemies from behind, that guy very well might try.