More information specific to Easter (Wikipedia):
"The main Christian moving feast has been Easter. Several different ways of computing the date of Easter were used in early Christian times, but eventually the unified rule was accepted that Easter would be celebrated on the Sunday after the first (ecclesiastical) full moon on or after the day of the (ecclesiastical, not actual) vernal equinox, which was established to fall on 21 March.
The Catholic Church made it, therefore, an objective to keep the day of the (actual) vernal equinox on or near 21 March, and the calendar year has to be synchronized with the tropical year as measured by the mean interval between vernal equinoxes. From about AD 1000 the mean tropical year (measured in SI days) has become increasingly shorter than this mean interval between vernal equinoxes (measured in actual days), though the interval between successive vernal equinoxes measured in SI days has become increasingly longer."
The thing is, our year is approximately one full solar orbit of our planet, I believe. And a month is a moon's phase. Only, there aren't exactly the right number of days to marry the two neatly, so it's all completely jumbled. Add in the fact that the Jews originally used a different calendar to work it out and the fact that no matter how you do it you lose consistency with months, seasons or the solar year itself, you're pretty much screwed. So even though the day you die is the same, its anniversary is a rather arbitrary date - whether you're Jesus, the Invisible Pink Unicorn, or an anime fan.
R
