In the Winter people in the Northeast (NYC, Boston etc..) want to go someplace Warm (makes sense), Florida, Mexico, Carribean etc.. The exception is the ski and snowboarders who want to go to Colorado etc.. Hence all the Seasonal nonstops from EWR
to Montrose, Vail, Steamboat Springs on CO
and daily nonstop service from EWR
to Vail on AA
. With high demad comes higher prices, in the Summer these destinations are cheaper.
In the Summer people want to go to California, Bermuda, Alaska, Canada, Europe etc.. So during the Summer fares on these routes are higher.