Christmas -- December 25The date is tied very deeply into the natural cycle of decay, rebirth, and renewal. Easter always falls on the Sunday following the first full moon after the vernal equinox.
Some people say the early Christian church tried to "co-opt" the earlier pagan religions by finding ways to use their holidays as its own, others see in it a reflection of Christ's teaching, in which the Ten Commmandments of the Old Testament were updated to the Two Commandments of the New Testament: they are not "co-opting" they are adding a new layer of sophistication over an existing layer of belief in the Divine.
St. Patricks Day -- March 17
Easter and it's associated holidays - random date based off of the moon with the idea that it will always be on a Sunday.