Graubünden has essentially two climatic regions. In the lower Rhine Valley (Chur, Heidi-Land, etc), there are normally four distinct and equal seasons. Up in the Engadine, there are four seasons as well, but their distribution is heavily skewed toward winter. Snow can remain until May, and after a brief spring and summer you may experience flurries or frozen dew in August. So even if traveling in summer, pack some warm clothing, like a sweater and a windbreaker. The fall colors are well worth a trip -- perhaps not as spectacular as New England, but very inspiring.