October to March is the season — cool, dry, comfortable for walking. December and January nights can be cold but the days are perfect.
April through June is brutally hot, with temperatures regularly climbing past 40°C. The exception is the Urs festival in late May or early June, the saint's death anniversary, when hundreds of thousands of pilgrims pour into town and Ajmer is at its most extraordinary — also at its most exhausting, with every hotel booked months in advance.
The monsoon (July to September) is short but useful: the dust settles, Ana Sagar fills up, and the hills around the city turn briefly green.