Spring Break can refer to both university and lower level (high school and elementary) school break periods, usually one week in duration during early Spring. The actual dates vary from place to place and school to school around the US and are staggered across the entire month of March. Some lower level schools in the Northeast US get the third week of February off too, some private schools get the weeks off before or after Easter (this year Mar 23). From my experience Spring Break ski crowds are most noticeable in Colorado, particularly the Summit County areas and a few other notables such as Vail, Heavenly and perhaps Park City. Not sure about Whistler. But in any event, US ski areas during Spring Break periods don't usually get as intensely crowded as what may occur during the week between Christmas and New Years. I wouldn't let the possibility of a visit coinciding with March Spring Break deter you from coming to the US to ski. March is a marvelous month for sunshine and generally good snow in the US West.