The generic answer is 14 days after the 1st day your last period began on. No one should rely on this because there are many variables, all it takes is for your cycle to be irregular by 1 day on 1 month to get pregnant. Do you know what everyone who uses the rhythm method of birth control gets called sooner of later? Parents.
This is called 'Fertility control' for contraception, but it can also be used for calculating your fertile time for conceiving.