To me, the primary considerations are pretty simple:
1) Is there ANY chance of freezing? If yes, don't open.
2) When can you reasonably expect your water to be warm enough to swim? I use solar heating so a good patch of sunshine and we can swim in early May in N-Central NJ.
3) If it's too cold to swim, and going to be, then you'll want to delay opening as long as possible--why burn electric and chlorine if you don't have to?
4) OTOH, if you are seeing algae growth, you may want to open so you can filter and shock heavily.