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Astronomically it can vary by a few days but is usually on the 21st or 22nd December (in the northern hemisphere obviously) - it's quite difficult to observe the exact moment so most observe the sunrise. An almanac will give you the precise date each year.
I'm not aware that Avebury has any specific alignment to the winter solstice (unlike Stonehenge), but if you go there you wil not be alone!
I think Yule though was in the past a (literally) moveable feast, based on the lunar calender. The two have of course become synonymous.