Pahalgam's main season runs from May to October. The best months for trekking are June and September — after the worst of the snow has cleared and before the pre-monsoon heat builds (June), or after the monsoon rains and before the early snow closes the high passes (September). July and August bring the Amarnath pilgrimage, which floods the town with hundreds of thousands of pilgrims and significantly stretches accommodation and transport. If you are coming specifically to trek, avoid the Amarnath peak weeks (typically late June to mid-August).
Early October is beautiful — clear skies, turning trees, and a sudden quiet as the pilgrimage season ends. The river is lower and calmer, and the trail to Aru and the Kolahoi base camp is still open. From November, snow closes the higher passes and most trekking routes become inaccessible. The town essentially shuts down from December to February.
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