Simple answer: When the snow melts, or when skiers and snowboarders stop coming.
Ski areas close most years with plenty of snow till on the trails.
The operators decide to stop running the lifts because their guests have stopped coming, lured away by other pursuits that capture their interest in springtime.
These include other recreational activities like golf and bicycling, and household chores like lawns and gardens.
"If they're mowing the lawn," says Tom Meyers of Wachusett Mountain Ski Area in Princeton, Mass., "they're not thinking about skiing or riding."
Bottom line is that the decision to close, most years, is an economic one. It costs money to run a ski area, and without paying customers, there's not revenue coming in.
Winters like this one, 2010-2011, with lots of snowfall through mid-February, good trail conditions well into March, and then another series of snowstorms combined with cold temperatures, skiers and riders keep their interest up, and keep coming, so resorts tend to extend their season by a few weeks.
Question: When do ski areas decide to close?