Mandi is at a lowish 800 metres for a Himachal town, which means summers can be hot — temperatures climb into the mid-30s in May and June, and the air in the valley sits still and heavy. The town is at its best in March–April (warm days, the rhododendron in flower on the hills, the Shivratri Fair if dates line up) and October–November (clear post-monsoon air, snow returning to the high ridges visible from town).
The Shivratri Fair in February or March (dates set by the Hindu lunar calendar) is the single best time to visit if you can plan for it: a week-long procession of village deities, hundreds of palanquins arriving over the mountains, the whole town strung with bunting. Hotel space disappears six months in advance.
The monsoon (mid-June to early September) is wet but not unpleasant; the river runs full and the temples look dramatic in low cloud. The roads to Rewalsar and Prashar can wash out after heavy rain. Winter is cold (sub-zero at night in January) but rarely snowy in town itself; the high passes north of Mandi close.