Thanks for the replies, I appreciate the recommendations, and so far, the consensus seems to suggest March, and maybe late Feb. Normally, I do like spring skiing in March when I go to Colorado. How late in March is good skiing without the snow becoming slush? Is early, or mid-March best? Thanks!
I think the records show that early March offers the best opportunity for great snow at every major resort in the US Rockies, Big Sky included. It's a combination of accumulated snow pack plus frequent spring snow showers/storms.
I ski Big Sky late December to early April and have always found enough snow to enjoy the trip.
I would say February to early March. Big Sky has a substantial amount of terrain facing the sun, including the majority of runs off Lone Peak, though high on the mountain snow still preserves better than other places with that exposure. The addition of Moonlight Basin has made March somewhat safer than before, because nearly all of Moonlight faces north.