Just back from a wedding in Chamonix. Even at that elevation, it was still 30°C and over. On the plus side, the weather was beautiful and the reception lavish. Go to a French wedding, if you're asked. You will not regret it.
[nights] In fact, this is the route I took for a day out with my (then) fiancée and 8 month old son in a rented Twingo. The views on the road down to Martigny were phenomenal.