I very much doubt that there will be any cabana sales for the upcoming spring and summer seasons. As the weather gets nicer and nicer (and the daily temps get higher and higher) they will have no problem filling those cabanas at regular price. If anything, I would say that it's quite possible prices will increase instead of decrease as we get closer to summer.
We reserved an Aquatca cabana for March trip several months ago. We had hoped to secure our March date at a discount on the Black Friday sales, but no such luck. We got our (absurdly expensive) DC cabana on a New Year's sale in early January, which surprised me greatly ..... but I haven't seen any sort of discounts since.