Are you also using a solar cover to retain the heat you've already gotten? I have used my heat pump to heat my spa when the the air temps were in the fifties so the heat pump will work in your temps. BUT, you should also use a solar or other type of cover to hold the heat in.

I agree with moneypit.....retaining that heat with a solar cover will go a long way...otherwise you might just be spinning your wheels. The heat pump will bring up the temp just take longer at cooler temps. You can run it with the solar cover on without an issue. Let us know your results of how the heat pump produces in cooler temps.

I definitely agree with the other members about the blanket to hold your heat in the water.
However, if you don't have one just run your pool 24/7 until the party.
Heat pumps will draw heat out of cooler air but you must keep them running or risk losing your gain overnight.
I'm certain that if you run from tonight until the party on Sunday the water will be at a great temp.

See ya,
Kelly

Thank you all so much - I feel much better now! I do use a solar cover and last night only lost 2 degrees even without the pump on, so I figure I'll keep it going until the party and the kids can swim in nice toasty water, even if it's cold outside!

A little update on how it worked - Saturday was very cool (low 60s) and absolutely no sun, rained hard most of the day - I had the heater going the whole day and gained 4 degress- up to 83. Overnight, I kept it running, trying to get the temp up a little more, and it lost 2 degreees (it was a cold night). Luckily, Sunday ended up sunny and fairly warm (70s) and by party time it was 86 and perfect! Thanks again for your feedback!