That's a really good point (although we do have spring and autumn/harvest festivals too), especially taken in conjunction with the point about the direction of the avenue. But then the post holes in the car park might lead us to another conclusion.
I think it was used all year myself.
At least these telly programmes don't have "druids" doing sacrifices and all that stuff, like they seemed to when I was a kid.

In Germany Mayday is a much bigger event than here. They light fires all over the country and in some parts drink for 5 days solid...

Midwinter seems much more likely to me and could be compared with winter festivals of light all over the world. Seem to remember something about the layout at Stonehenge being geared to the winter solstice sunset - that being exactly opposite to the summer solstice sunrise. Why would the ancients want to celebrate the longest day? Surely the time to celebrate summer is when the harvest is safely in. Midwinter festivals are fire festivals and designed to stop the sun's decline and bring it back. You might like to glance at http://www.megalithic.co.uk/article.php?sid=2146412213