(note - Winter Solstice for US, however December 21st and a half is Summer Solstice in Australia...)
Here's a picture summary (note - be sure to check out the source links at the bottom for other cool Maths related stuff as well)

Happy other upcoming Holidays too (Christmas, Hannakuh, Festivus, Kwanzaa, Newton's Day, etc).
Sources -
Source Maths Article - The Length of a Day
Math's Website