Kelowna has the sunniest climate of any major city in Canada. Summers are hot and dry — July and August regularly hit 35°C — and the lake is warm enough for swimming from June through September. This is the peak season: beaches fill up, wineries are busy, and accommodation prices peak. Book well in advance for July and August.
Late September and October are arguably the best time to visit. The harvest is underway in the vineyards, the temperatures drop to comfortable levels, the lake is still swimmable, and the tourist crowds thin out. The Okanagan wine harvest is one of the great seasonal events in Canadian food and drink.
Winter brings skiing at Big White Ski Resort, about 55km southeast of the city. November through March can be cold with some snow in the city itself, though nothing like the Pacific coast winters.
Spring (April and May) is quiet, with the orchards blossoming and the tourist infrastructure reopening after winter.