There's no date or time which is objectively special, of course. Historical accident has simply given many societies a common period in the middle of Northern hemisphere winter in which we band together to banish the cold and darkness for a time, to remember that light and warmth will come, and to take some time (however brief) away from the constant cruel work that survival requires.
Of course, many people don't need a festival like this any more, and it's easy to forget why it's there. Various religions have piggy-backed off this practice to celebrate specific things at the same time. The Northern European cultures in particular were extremely infectious and have exported the practice to many geographic locations where it's neither cold nor dark at this time of year. For many people, the "constant cruel work" is no longer either constant or cruel. But at least in the UK we have long memories, and some things are worth remembering!
(Also, go and read "Hogfather", by Terry Pratchett. As I seem to remember Yudkowsky said, though presumably in much better wording, Terry Pratchett's death is all the reason you could need to resolve to end death forever. Hogfather, in particular, I think is one of his crowning works and I sometimes choke up a bit just thinking about the book.)
Of course, many people don't need a festival like this any more, and it's easy to forget why it's there. Various religions have piggy-backed off this practice to celebrate specific things at the same time. The Northern European cultures in particular were extremely infectious and have exported the practice to many geographic locations where it's neither cold nor dark at this time of year. For many people, the "constant cruel work" is no longer either constant or cruel. But at least in the UK we have long memories, and some things are worth remembering!