snofun for the win.
Wait as long as possible. Your trip includes Easter, so there may be some spring break overlap, but still -- it's April, and most people are on to baseball by then. (Fools.) Deals will abound as you get closer, at least for lodging and lifts, and possibly for flights as well.
Some data points and ideas...
From a cost-effectiveness perspective, you may want to consider that both Squaw and Alpine will likely be running spring pass deals by late March. Prices haven't yet been set, but should remain under $150, and the amount should be transferable to a full pass next season.
Snowbird is a safe bet to remain open for a while. (We closed out last season there on June 1, and it remained open for a few weeks after we left.) The other Utah resorts usually close by then, though.
The only times I've ever skied Mammoth have been over July 4th weekends. April is supposedly fantastic.
A shot of my boys on the back side of Alpine in April 2006: