Even though, unlike Ladakh (see Best time to visit Ladakh), Spiti Valley is accessible throughout the year, however only a couple of months can actually be considered as best time to visit Spiti.
This is due to the fact that while Spiti Valley itself is mostly in rain shadow area and is unaffected by monsoon season, both the approach roads leading up to it, regularly close during monsoon season, due to landslides. Add to that, Kunzum La remains open for less than 5 months (from June to mid-October), limiting the prospect of those, who wish to do the complete Lahaul-Spiti circuit.
So which are the best months to visit Spiti?
June and September are the best months to visit Spiti Valley and here is why:
Spiti in June: In June even though you will have to wait for Kunzum La to open, before you can begin your journey. And road conditions between Gramphoo and Kunzum La are at its worst. Roads are mostly stable, and even if Kunzum La – Manali route closes due to avalanche or landslide, you can return via Shimla route.
You can also see plenty of snow between Kunzum La and Rohtang Jot and weather till Kaza (if you are going via Shimla route) is mostly pleasant. There is also a lot of greenery till you enter Spiti Valley, and almost all hotels and restaurants in Kaza are open.
The only downside is, you might end up missing out on Chandertal Lake, if you are going early in June or in case road opening gets delayed.
Spiti in September: Even though you will hardly find any snow in September, it is actually a much better time to visit Spiti than June, due to the fact that monsoon season is over and roads are open and stable. Only minus point about traveling to Spiti in early-September is that road condition will be bad at some of the places, due to monsoon fury and road reconstruction process will be in full swing.
Of course if you are an adventurous soul, who is ready to take the chance of getting trapped in Spiti or coming back from halfway due to landslide, then there is no problem with traveling to Spiti in July or August, when almost all of Himachal Pradesh witnesses heavy rain.
However, it is my recommendation, that it is best to visit Spiti when the weather is good, both roads are open and there is minimal chance of nature’s fury and unfortunately for us travelers, in case of Spiti, it is only valid for June and September.