Leap of faith

Today is Leap day, February 29 of a leap year. That is an odd name for it, since we do not leap over it as we usually do. It should be called "sit day"?
But why do we do it?
The actual length of the year from one vernal equinox to the next is 365.2424 days. If we use 365 days a year, in 100 years we are behind by 24 days. So the seasons creep forward. Spring comes later. It does not come out even.
So every 4 years we add a day. That averages to 365.25 days.
That is close, but not perfect. So we add a new rule: Every century we skip the leap year. So, 1700, 1800, and 1900 were not leap years. That brings the average to 365.24. That is closer.
Every 400 years, (when the century is divisible by 400) we skip skipping the leap year. 2000 was a leap year. That adds in 1 day every 400 years, bringing the average to 356.2425. That is pretty close.
There is a proposal to put in a leap year in the year 4000, but it is not official, and there is little urgency to the idea.
Changes in the Earth's orbit and day length can make these adjustments tricky. About every 18 months, we have a leap second: We add 1 second to keep the stars lined up better.
With all of this, the location of the stars in the sky as the years go by will still drift a lot.
Why is it that the calendar is adjusted for the seasons, but not the stars? To keep Easter near the Vernal Equinox. That is the "faith" part of my title.
Take a look at the extensive article on Wikipedia on this subject. There is much more.