Avoid July, August and the National Day holiday. Those are the two windows when domestic travel peaks — the summer school holidays, and the first week of October — and on these roads that means traffic, full guesthouses and slower driving than the itinerary allows for. Outside them, the season runs long enough that you have real choice.
The two windows to plan around
July and August. The Chinese school summer holiday puts a very large number of families on the road at once, and the plateau is a favourite destination for exactly that. The roads out of Chengdu and the main scenic corridors get genuinely busy.
The National Day holiday. The first week of October is the other peak, and it is more concentrated: everything happens in a handful of days, at the same time, on the same roads. If you can move your dates by a week on either side, do — the difference is not subtle.
Neither of these is about bad weather. It is about company.
What that leaves
Most of the rest of the year. The plateau is a big place at altitude, so the character of a month depends on where you are going and how high: spring comes later up here than it does in the valleys, and autumn arrives earlier. That is worth holding on to when you look at a general map of China's seasons — it does not apply neatly to a grassland at 4,000 metres.
Winter is properly cold on the high ground and much quieter, and the snow is part of what the landscape looks like then. It is not a season to write off, provided the route is chosen for it.
The practical advice
Book around the two peaks rather than inside them, and if your dates are fixed and land in one of them, build in slack: shorter driving days, accommodation arranged earlier, and a willingness to change the order of the trip. Crowds on this kind of road are less about people and more about time — everything takes longer than you budgeted.
For what a given route looks like month to month, the route pages are the place to look — each one is written for a specific trip rather than for the region in general: every route I've written up.
Questions I get on this one
When is it busiest?
Should I avoid the school holidays?
Is winter a bad time to come?
Does altitude change the seasons here?
Written from driving these roads through the year rather than from a guidebook. Related: how long the drives are · altitude · all essentials