Easter, which commemorates Jesus's resurrection--for those who believe that--falls on a different day every year because it is based on the lunar calendar. It falls on the Sunday following the first full moon of spring. This year, the first day of spring was March 20, the full moon will be on March 29, and Easter on April 4.
Christmas, on the other hand, is supposed to celebrate the birth of Jesus--again, for those who believe that. It falls on the same day every year, December 25, a few days after the winter solstice.
I understand both days are quite holy in the Christian religions. I don't know too much about the religious aspects, as I grew up Jewish and am now a secular humanist. But why is one calculated per the lunar calendar and the other per the solar?