Hi Shuhadeep, Spiti valley is accessible from Shimla side throughout the year however from Manali; it is accessible only for 5 months, from June till September. So if you are planning from Shimla, from June till September, you can complete the circuit by exiting from Manali. However from late October till early May; you can only drive up to Kaza from Shimla and will have to return the same way. There is no high altitude pass between Shimla and Kaza however when it comes to Manali; you will cross two high altitude passes, Rohtang and Kunzum Pass. Both these passes remain close from anytime in October till May (depending on the weather) so a journey from Manali to Spiti valley can only be done from June till September.
Anytime actually. I have been during June and July also. For clear sky, you can visit after Sep and if you want to witness White Spiti, then during Dec.
April to mid-May: At this time, the snow starts to melt. ...
Mid – May to mid – July: This is the real peak season in Spiti when the valley welcome hordes of tourists who wish to get respite from city heat.