Jammu is at its most pleasant from October to March. The city sits at roughly 300 metres elevation at the foot of the Shivaliks, which means summers (April to June) are genuinely hot — 40°C or above — and humid enough to be uncomfortable. The monsoon arrives in July and runs through September, bringing heavy rain and disrupted roads.
Winter is mild and dry by local standards, and the city functions as the winter capital of Jammu and Kashmir during this period, adding administrative energy to the usual pilgrim traffic. If you are planning a Vaishno Devi pilgrimage, January and February work well for the hike but nights at altitude are cold.