I think because it is a religious holiday. Christmas is as well but to be honest, there is more going on with Santa than Jesus now a days. Plus there is like 15 other religious figures born on December 25th... but that's a whole new can of worms. --- Before you ask a question, please look it up yourself first. Wikipedia.org is more reliable than you think.
It presumably has nothing to do with religion, since two of the dates are saints' feast days. Probably it was just more convenient to pick a holiday that occurred on a fixed day in the month. Also, the Orthodox used a different date for Easter, so that could have caused some confusion, maybe.
From: LeetCheet | #006 VividAxis posted... "Only one date for each month." You should pay attention to the bold part. And if you still don't understand: The first Sunday isn't always on the exact same date next year.
I know that. I can calculate that exact moon phase with a three step algorithm. I'm fully aware it doesn't happen on the same day each year.