Plead ignorance and go and have a good time.
and don’t piss off any walkers please ta
It’s 1 September, in answer to your question http://www.chamonix.net/files/Mountainbiking/planVTTEn.pdf
I’ve posted on here with this same question. I’ve been told it’s not really enforced, but if everyone rides with that assumption, that will generally be A Bad Thing. So I’m not going to ride the valley trails on that basis. Do a little research, play with google earth, pick up an IGN map, talk to people on here and you’ll see there’s rather a lot that falls outside the scope of the ban. You gotta be prepared to ferret it out though.