Spring (April and May) and autumn (September and October) are by far the best times. The Hisar valley is hot in summer — temperatures often push above 35°C in July — and the open fortress hill has almost no shade. Winters are cold, occasionally snowy, but rarely severe.
If you can, time the visit for Navruz (around 21 March), when Tajiks celebrate the spring equinox with music and family picnics. The grounds of the fortress are a popular spot for the day's festivities.