Spiti Valley remain open whole year but there are certain parts that remain closed like losat to kunzum pass. - According to the locals, the months of June and August get the best temperatures, which is exactly when most tourists flock to the valley. During these months there are some major festivals held in the area so this would be a good time to visit to partake in the festive celebrations. - The best month to visit Spiti is from June to October. The mountain passes are open and the weather is moderate. - If you want to trek then the best time to go would be during July and September as this is the time when the weather is great and the valley is beautiful.
Hey Prem, People usually prefer going from June to October as the temperatures don't get too low. You could also go before June but from Shimla side, as the Manali road gets opened by the first week of June.
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.