Spiti valley is accessible by road throughout the year.
There are two ways to enter Spiti valley- one via Manali and the other via Shimla. Usually the best time to visit Spiti are the summer months of May till Mid October when the temperatures are great and there is no snow or ice on the roads.
In case you plan to start the trip from Shimla and exit from Manali, you should plan in the summer months, since there are two main passes enroute i.e. Rohtang pass and Kunzum Pass which remains closed due to snow in winters. The route starts clearing up in the month of June.
If you plan in late October or later, you can take the road from Shimla upto Kaza and you would need to exit via the same route.
April to Mid May is a good time to be at Spiti, however the route via Manali is yet not available. Snow starts clearing up, the guest houses start opening up, the peaks are yet snow studded for those perfect shots and roads by now become bearable.
Mid-May to Mid-July is one of the best times to be at Spiti. The days are pleasant. Hotels are running full fledge. Chandertal is accessible post June. Loads of campers and trekkers prefer this time of year to be here.
Mid-July to August is not really a great time to be here. Monsoons are in flurry and roads are in horrible state. Offlate Spiti region has started receiving good rains in these months despite being in Trans Himalayan region thats generally rain shadowed.
September to Mid-October is a great time to be at Spiti. ALl roads are open, running, connected and accessible, be it the remotest village. Its cold, so carry woollen wears. Do not expect snow in this month. The peaks are mostly naked. You would also find the amazing Kinnauri/ Spiti apples in this month. Do not forget to taste/ buy them. They are the tastiest apples you would ever have.
Winter in Spiti is really harsh with the temperatures dropping to as low as -30° Celsius. Locals celebrate a lots of festivals at this time to keep up the spirits. People are generally house arrested in these months and they stock up food well in advance for the survival. But yes, this is the best time to spot the snow leopard or the wild ibex.