I don’t live in VT. My pool experience is from decades of living near MHT. We always opened Memorial Day weekend, but we never used it until mid June because no one wanted to go in 65-70° water.
I’m not getting near a pool in May without a heater.
I feel like our true seasons line up well with the astrological ones. Deep winter is gone by 3/21, consistent summer temps by 6/21, first 30s around 9/21, and the pack establishing by 12/21.