Most suitable time for visting Spiti valley is in month of June to September(primarily, summer months). Its one of the best time to visit as the temperature is pleasant and classy!
With almost no frozen roads, the summer seasons are the most desired timing for travelling.
However, winter in Spiti is really an arduous task, as temperature usually drops to -30° Celsius- which is almost unbearable. Plus, Manali-kaza highway is blocked in winters.
So, I would recommend you to visit Spiti in Summer season. Happy journey!
In the month of April to mid-May: At this time, the snow starts to melt. A number of roads are opened too like road from Kaza to Losar. However, road from Kunzum Pass towards Manali open only in the June month. This is the time when reaching Spiti is feasible and a number of adventure enthusiasts make their plans regarding tour to Spiti. This is an ideal time to admire snow-clad mountains and play in the snow. If you do not really mind low tourist count, can stay amid basic facilities and do not mind certain road blockages, you can travel at during these months.