Spring and autumn are the obvious choices. March to May brings wildflowers and mild temperatures (highs around 25°C); September and October are warm but tolerable, with the wine harvest in full swing.
Summer is hot — inland-California hot, 35–40°C is normal in July and August — and the wineries are crowded. Winter is quiet, sometimes rainy, but the vineyards have their own austere beauty when the vines are bare.
Three big events worth timing a trip around: the Temecula Valley Balloon and Wine Festival in early June; the Crush Festival at harvest in September; and the Old Town Bluegrass Festival in March. All sell out hotel rooms; book ahead.