The ideal time has started. April to June is very good though July is not bad and then Aug-early October. just check the rains in July. you'd probably want to start from Kaza or you can go to keylong and start from there. Best of luck for a heavenly and humbling experience of these majestic mountains that due to their barrennes seem to change colors with the shade of the sunlight. With them on your sides as you criss cross the Spiti valley, and sparse villages in between, you can be assured of an experience of a lifetime..