Your birthday creeps one day of the week forward each year and two around leap years. Here's why:
A common year has 365 days. 52 weeks is 364 days (52 x 7), so the year has 52 weeks plus one day as a bonus (364 + 1 = 365).
So for most years, if your birthday was on a Friday, the following year it will walk forward to be on a Saturday 🎉
Leap years have another bonus day on 29 February. The bonus day means that your birthday will move forward two weekdays instead of one around leap years, however, how your birthday moves depends on whether it is before or after that date each year.
Here's an example.
Birthday after 29 Feb:
Birthday before 29 Feb:
Perhaps someone told me this when I was a child, and I was keeping close track of birthdays but even if they did it never sunk in. Hopefully, the little person leaping a day will help you, like me now, remember forever.
It was pointed out to me that the changing weekday of your birthday is a very good thing—would seem a little unfair if you were stuck with a Tuesday all of your life.