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