The best months for Lhasa are May, June and September. Days are warm and dry, the skies are clear, and the great festivals — Saga Dawa in May–June, Shoton in August — bring the city alive. July and August are the wet season but the rain falls mostly in short afternoon storms; the upside is that the mountains and grasslands are at their greenest.
Winter is cold but not as bitter as the altitude suggests — Lhasa sits in a sheltered valley and the sun is intense. December through February offer the lowest prices, the fewest tourists, and the highest concentration of pilgrims, who come during the off-season when they can. The cold inside unheated buildings is the real problem; pack a heavy down jacket and expect frozen mornings.
The Tibet Travel Permit, which all foreigners need, is sometimes suspended around politically sensitive dates — the anniversary of the 1959 uprising on 10 March is the most reliable annual closure, often running through into early April. Check permit availability before booking flights.