EQN -Earned Qualifying Nights?

Not much interest in those, how does the new category thing work? IE: I want to use my two current nights @ the Marriott in Oakland right now its a cat4 $115 a night, searching in april reveals $250 night rates, as I assume it takes in account the moving up of categorys maybe? Regardless, if I book now, when do I have to use the room by?