Season geography
Across New Hampshire's featured cities, typical openings spread from Jun 12 to Jun 12, closings from Sep 1 to Sep 3, and the unheated season runs 81–83 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.