It's a standard question: "how early should I book for a ski trip?" The easy answer is as early as possible...with an exception.
The general rule of thumb is if you know you want to go skiing, start planning early. Sure, you could save a good amount of money, but that's not the primary reason. Hotels and flights to ski areas get booked early. If you want to have any options for your travel and lodging, you need to start well ahead of time. However, you may want to wait closer to the time if you are planning during the beginning or tail end of the season.
Lift Tickets
Do you know which mountain you're visiting? Look into lift tickets as early as possible. Ideally, investigate them before the current season ends.
The earlier you buy your lift tickets, the cheaper they tend to be. Start looking at ticket prices as soon as you know your destination.
A lot of mountains sell season passes for next season at the end of the current season. These early-bird season passes are often cheaper than just a week's worth of lift tickets purchased later. Vail resort's EPIC pass program is an example of this. It goes off sale when the season starts so it's best to pick this up, especially if you are planning a ski vacation to Colorado.
If the last season has already ended, you'll often find further "deal dates" sometime around mid-summer, labor day, and opening day.
Start investigating ticket prices as soon as you know where you're going. It could save you hundreds.
Hotels
Hotels and condos book quickly during ski season. Those that don't completely fill, get more expensive as the holidays approach. If you're visiting a popular ski resort between Christmas and New Year, expect rooms to be booked a few months in advance. Spring break's another time where you'll struggle finding a room. Book early, or you might be stuck with few options.
Flights
Flights also book quickly. Flights to top resorts at peak times can get completely booked over 6 months in advance. If you are set on a location, investigate early and book your travel.
The Exception
If you are planning to visit a resort in the very beginning or very end of the season, booking early might backfire.
Resorts can delay opening dates if the temperature doesn't drop low enough for snow. Booking in advance for a vacation in November could potentially result in a ski vacation without snow. It's rare, but it happens.
Closing dates and late season weather are also highly variable. If you make reservations for April, you might end up with more dirty water than snow. On the upside, you can usually ski in shorts and a t-shirt.
Resorts tend to give large discounts for late season tickets and rentals. If you're planning a trip in the last 2 weeks of the season, then waiting to book lift tickets might actually be a good idea. If you're curious, give your chosen resort a call and see if they have any late season deals. That said, you should still consider purchasing hotels and flights early, just like any vacation.