I sometimes run an earlier June trip...
There are 3 reasons for this;
1. There is a festival of motorcycles which takes place in Bourg St Maurice, normally "in mid-June". It would be nice for my tour to coincide with this. YES - there was the first Trans Vanoise rally in June 2004. We were there. By the organisers' own admission, the event was poorly attended. However the tourist board are pushing this again for 2005 and beyond, and hopefully it will grow, year on year. Why do this? Well, there are more bikes, and more events and entertainment going on, but they also supply local french guides who lead ride outs - takes the strain off me a bit, and offers my guests more choices. When in Bourg, I don't want to ride EVERY day as there are numerous other exciting options available, but for real petrol heads, there's a guided riding option practically every day if you want it - at all paces.
(in 2005 the bike rally was even smaller than 2004. I don't think I'll bother making my tour coincide with this anymore)
2. In some year's we've managed to catch "La Fete de St Jean" in Bourg St Maurice. It's usually held in mid-to late June, and is a jolly good party. The whole town turns out, and the Sapeurs Pompiers (Fire Brigade) set up a bonfire. Live music (very french style) and an outside bar, often dictate a crazy, fun evening.
3. The earlier we go, the more chance of lots of snow remaining on the high mountains - this doesn't detract from the riding at all because the roads are usually clear and dry - but it makes the views oh so pretty!
Picture - 2002 group parked up just outside Chamonix Mont Blanc