Season geography
Across Massachusetts's featured cities, typical openings spread from Jun 5 to Jun 14, closings from Sep 2 to Sep 17, and the unheated season runs 80–103 days depending on where you are. Elevation, coastal water, and urban heat all pull real dates around, which is exactly why a state-level answer isn't enough.
For your exact town, the pool season lookup takes a ZIP code. Opening day itself is a chemistry job — the dosing calculator turns your first test reading of the year into exact amounts.