Depend of the region you would like to ski; in general, the ski touring season will start from February till June. The best season for famous destinations is as follows:
Mt Damavand & Dobarar Chain: March & April
Zardkuh mountains: January-March
Alborz Ski Region (Shemshak, Dizin & Darbandsar): February & March
Alamkuh & Takht-e Soliman chain: April-June