When to open? Go by low daily temp or what?

[Watermom]

As long as you are getting temps below freezing at night, wait to open. When you are staying above that and having warm days, just keep an eye on your water. We kind of thought around here that as long as your water temp was below 50 degrees, you didn't really have to worry about any algae. Al (Poconos) and I both found out this winter that isn't necessarily true. We both had a slight streak or two of algae and both of us had temps below that. A quick shot of bleach took care of it. I understand your reason for not wanting to open only to have to keep cleaning a pool that you can't swim in. There really isn't a hard and fast answer as to when to open. Go ahead and reach under the cover and pull out a sample and test. As long as you are maintaining a chlorine reading, you should be OK for a bit.
Good luck.

Watermom

[Poconos]

I don't cover my pool anymore and when the ice melts I just keep an eye on the water. Watermom was right in that I did have a slight algae bloom when the water was a little above 40 I believe. As for equipment, I wait until the temps won't hit the freezing damage point before hooking things up. Solar stuff won't get water until I'm REALLY sure they won't freeze. Be aware that on clear cool nights the solar heaters will become good radiators of heat and freeze first. With a clear sky they are looking at cold space. Before you fire up the equipment, if you have an algae problem, you can add chlorine (bleach or whatever) and circulate by any means, even a simple submersible cheapo pump.
Al