Think more about the water temperature. I suggest that you open before the pool water reaches 60 degrees. After the water reaches 60, algae likes to take hold. Even if you can't use the pool until the weather gets better, it's easier and cheaper to avoid the algae and open before it can get started. I am in a similar climate and for us, I start checking around tax day, April 15. (It's just a handy way to remind myself!)