In the sentence It was a usual day should I say a usual or an usual?

I would not say either; it is better to say It was a typical day, or It was an ordinary day. A usual day doesn't sound quite right to me.

Getting to the heart of the question: There is a general rule that if the word following the indefinite article (a) starts with a vowel, then you should use an instead of a. But that is not true in every case. Most 'rules' in English have many exceptions. Probably, most words beginning with y will take a instead of an.

• The Cherry Blossom Festival is a yearly event in Washington, DC.
• They gathered for a Yuletide feast.

Words beginning with u can be tricky. If the u has a sound similar to y as in year, then it would take a instead of an. If the u sounds like the beginning of under, then an is taken.

• The students decided to take a unified approach to solving the problem.
• Fearing an undertow, John stayed out of the water.
• Solange wore an ultra-chic gown.

HERE IS THE TRICK behind the differences in usage above. English speakers want to avoid certain quirky, difficult or unpleasant sounds. If I want to say She has a umbrella, you can hear that there is weird feel to the phrase, and a need to make a gutteral stop, a break in the sound, in order to say the two words, a umbrella, separately. That stop is not a natural or comfortable pattern for most English speakers, and this is why an exists in the first place. It is about the sound, not the spelling. When I say an umbrella, I can continue producing sound with my vocal chords, and the 'n' separates the words. For a couple of special reasons, the problem doesn't exist if the word is yearly, yellow, Yuletide, unicorn, utility, etc. But it does for most other words beginning with the other vowels.