October to March is the only sensible window. December and January nights can be surprisingly cold (down to 6–8°C) but the days are warm, dry, and clear — perfect for walking and for boat rides on the lake.
April through June is brutal: temperatures climb to 40–43°C and the heat hangs heavy over the white-washed city. Avoid unless you have to come.
July to September brings the monsoon. The Aravalli hills turn briefly green, the lakes fill, and prices drop sharply — but rain can be heavy enough to disrupt sightseeing. The Mewar Festival in late March or early April (timed with Gangaur) is one of the best in Rajasthan: parades, dancing, and a procession of decorated boats across Lake Pichola.