Jammu and Kashmir spans three very different climate zones, so timing depends on where you are going.
The Kashmir Valley is best visited from April to October. Spring (April–May) brings tulips, cherry blossoms, and fresh greenery — one of the most beautiful times in Srinagar. Summer (June–August) is the main tourist season and gets crowded; evenings remain pleasantly cool. September and October are arguably the finest months: the crowds thin, the weather is crisp, and the chinar trees turn gold. Winters are cold and grey, with heavy snowfall blocking roads to the wider valley, though Gulmarg stays open for skiing from December to March.
Ladakh has a short accessible season. The roads from Manali and Srinagar are typically open from late May or June until October. The best months are July and August — clear skies, accessible passes, all roads open. May and June can be fine but some high routes remain closed. September is excellent and less crowded. In winter, Leh is reachable only by air and temperatures drop to -20°C; the famous Chadar Trek on the frozen Zanskar River is a winter-specific adventure for those prepared for extreme conditions.
Jammu and the southern foothills can be visited year-round, though the summer heat (May–June) is fierce. The Vaishno Devi pilgrimage is busiest around Navratri in October and March/April. Patnitop and the hill stations above Jammu are pleasantly cool in summer.
Monsoon rains (July–August) affect the Kashmir Valley, making the roads slick and adding leeches to trekking trails. Ladakh, being in the rain shadow of the Himalayas, stays largely dry during monsoon — which is one reason July and August work well there.