Some time in the mid-70s (no quite one is sure of the exact year, maybe 1975 or 76), initially at Whiteface Mountain, up in Lake Placid, probably the coldest mountain in the east.
After a year or two, we switched to Pico (then, a small independent mountain, now, owned by Killington) since it was a bit warmer and family friends had a condo up there. Spent a few years there, and then switched again to Mount Snow for the shorter drive (grew up in North Jersey).
Our family vacations were all ski vacation. For Christmas week and President's Day week every year, our family of four would pile into the car and drive to Vermont.