For the Great Hall (Whomping Willow, and Knight Bus) the word is Summer (or Q2). They extended it’s lifespan due to the pandemic. (The Amazon prime day Lego discounter sets are mostly all retired, save for the Cargo Train).
There’s possibly another few still being saved for a few months...
Assembly Square and Roller Coaster are speculated to retire Q1/Q2 2020.
I’m surprised at a couple Brickheadz still on sale.
This is all just based on rumours found online, nothing is factual, and set in stone, so take it with a grain of salt.