The region of Sapa has two seasons, summer and winter. The summer season goes from April to November - July and August are the hottest. If you are visiting Sapa during this season, bring t-shirts and a pullover for the day, and a jacket for the nights. In winter it gets really cold here, sometimes (December-January-February) it freezes. Snow is not a common phenomenon.
If you are going to do some trekkings, bring all kind of clothes. I was told that not far from Sapa is the coldest place in Vietnam, and not far from there the hottest. And do not forget your hiking boots!
Vietnam Travel - Sapa Travel Guide