In reply to owenel:
Varies every year, in recent years most likely in June. Currently it is 'closed' as this has been a very dry year ( the driest I can remember for many years) so natural rockfall is abundant. A proportion of the rockfall on the route is caused by others higher on the route - whilst anyone can accidentally knock rocks off the high proportion of novices on this route seems to exacerbate the problem. So if the freezing level is fairly low and you go at a time of day when fewer people are on the route you can sway the odds somewhat.
As with any route in the alps, check for local up to date information in the local guides office as conditions of routes can change dramatically in a very short space of time.
We were on the Peigne a few days ago when two rockfalls swept down our intended route and the whole Aiguille felt like it vibrated. We chose to do a different route nearby. Some groups arriving later chose to carry on !?!