Here is just a bit on these mythologies and ancient holidays from various sources (namely my encyclodpedia).
Greek Mythology
Indian Mythology
(Note: Indian mythology differs from greek mythology in that it is still worshiped today as Hinduism. I mean no disrespect by calling this mythology. I simply call it that because this is not a complex enough page to do justice to a whole relgion that many people believe.)
Celebrations and Myths of the Solstices
(Ok, so "Winter Solstice" only appears as a holiday for one show. Still, we use the greeting all the time during December and I figure it's nice to learn a bit more about it.)