Why cold water is the trigger
The best closing signal isn't a date — it's water that holds below about 60°F. Algae is nearly dormant there, so a pool balanced and covered in cold water emerges in spring looking much like it went in. Close warm, and the cover becomes a greenhouse: weeks of algae growth with no sanitizer, paid for in shock and scrubbing at opening. Swimming may end in early fall; the close itself should wait.
Closing day, in order
Balance the water first — pH slightly high-normal, alkalinity and calcium in range (the saturation-index calculator shows where you stand), then a final shock a few days before the cover goes on. In freeze country, blow out and plug the lines and drop the water level per your pool type; in milder climates, many owners just run the pump on freeze protection. Label honestly: the details vary by pool construction — follow your equipment manuals over any generic checklist, ours included.
Frequently asked questions
Do Hawaii pools ever close for the season?
In Honolulu and Hilo, there's no real closing day — daily averages stay above 65°F in every month, so pools run year-round. The seasonal rhythm is lighter care in the cooler months, not a cover.
Why do the dates differ across Hawaii?
Because each city's dates come from its own NOAA 1991–2020 station records, and geography does the rest — elevation, latitude, coastlines, and urban heat spread the same milestone across weeks. That local spread is exactly what a national "open on Memorial Day" rule misses.