The Church of the Nativity in Bethlehem is one of the oldest continuously operating churches in the world. The structure is built over the cave that tradition marks as the birthplace of Jesus of Nazareth, and thus it is considered sacred by Christians.
What exactly is it that goes on in Mecca? As a non muslim looking on it appears to be an event, or more correctly a pilgrimage, a worship en masse, bigger then any other in the world. And it happens every year.