Maduo is viable June through September only for most travellers. The high-altitude plateau roads to the source lakes can be passable in May but are unpredictable; snow can arrive any month of the year at 4,300 metres. June and July offer the longest days and the best chance of clear weather, though afternoon thunderstorms are common in July and August.
The altitude (roughly 4,300 metres) is the primary limiting factor. Plan at least one day in Xining and ideally a stop in Yushu or another mid-altitude town before arriving in Maduo. Headaches and breathlessness on the first day are nearly universal; physical exertion should be minimal. Bring warm layers regardless of the season — nights drop well below freezing even in midsummer.