Actually - they had more to do with not being able to store foods much beyond this point, and new crops / beasts not ready for eating until spring arives. You don't slaughter a ewe before it has produced it's lambs or you won't have any sheep to eat the following year. The animals you slaughtered before Winter set in (they would otherwise have just starved to death for lack of food) are about to deteriorate with the warmer weather and go rotten (Winter was a natural outdoors fridge/freezer). What is the solution? Easy - have a big feast to eat your stored food before it goes bad, then starve for a month and a half until the new season provides fresh food.
Bog all to do with religion, if you think about it...

Mucky foreheads on the next day, and penitence are a completely different issue.

Quack!
GtD.