Is the constellation based on the day you were born or your birthday every year?

Constellations are calculated according to the solar calendar of the date of birth. For example, if you were born in the Year of the Tiger, you will be a tiger all your life, not a snake, because you are year of the snake this year.

So if your date of birth is Aries, it will always be Aries, and it will not change because of the different months.