Lost Interest As I Thought

So there we are, that was Xmas, and New Year, done & dusted, put to bed, with barely a whimper, never mind a bang.

I have decided that this year of 2009, I will celebrate the various Solstices, with Bonfires, Beer, a bit of a chat, (apparently this is what the Vikings,& suchlike were into, possibly a bit of lamb or venison in a pit of hot stones, and generally ignore all the faux christian celebrations.

So that'll be no Christmas, but the Winter Solstice on December 21st instead then. But the question is, how did those Vikings, and other heathen, know when Dec. 21st was - they didn't have a calender, or a diary...