I don't know what ski areas can do to change this, other than getting the AD Council to run PSA's for a decade: "Fact: Thanksgiving skiing sucks. Start an Easter ski vacation tradition and leave Thanksgiving for getting carbohydrate stoned and watching football. Paid for by the Ad Council."
People just have the mindset that skiing is Thanksgiving-February, and it seems once warm weather starts where people live (as opposed to where they ski), people stay home.
I do think ski areas TRY to change this. I've seen more advertising pushes for Spring Break, lots of Spring festivals at ski areas, that type of thing. But its hard to change the mentality.
I think the casual skiing population just has a really hard time grasping how great skiing can be in the Spring. I think they believe it all melts away with the first thaw, forgetting how long it takes snow to melt away when a snowpack 5-15 feet deep gets thawed only for a few hours each day, not to mention that it gets replenished with all the additional Spring snow.
The busiest day of the season for Wolf Creek was New Years. Wolf is a natural snow mountain with a TINY snowmaking capability for patching small holes in coverage, not even enough to WROD one run. Wolf had recieved 25 inches of snow in Mid November, NOTHING for a month before a nice 60" cycle week before Christmas, and NOTHING from then to New Years. Conditions at new years were adequate, but patchy, many of the expert runs and trees were still sketch,and the place was just mobbed with beginner 1-2 times a year skiers. Many I talked to had the idea that New Years/Christmas was the prime time to ski, many were quite surprised when I told them that the snowback continually grows until Mid March, and the later conditions were vertually always better than Christmas.
Once March hits, I notice the ski crowd starts thinning, with a temporary reprieve come Spring Break, followed by the ski area becoming a ghost town in April. I have to shake my head pretty much every year when the ski areas get a massive dump 3 days after they closed.
But sometimes it is not just about crowds- some areas have hard and fast closing dates mandated by the Forest Service- my understanding is Monarch is limited in staying open past the first weekend in April because of an elk migration that happens in April through the ski area.
Back to the question at hand- I think it is "really" ski season when enough natural snow is down to allow skiing on ungroomed, non WROD terrain. Last year for Wolf, ski season started April 8 with a 40" storm that opened most of the mountain before Loveland and Abay opened their 1 run WRODS.