Pool closing · State guide
When to Close Your Pool in Illinois
Last updated: July 15, 2026 · 18 cities covered
Closing deadlines across Illinois stretch about 16 days: Round Lake Beach needs the cover on first (October 2), Alton last (October 18), and the statewide median deadline is October 7.
Each deadline is the model's two-clock compromise — ten days after the water leaves the algae zone, capped a week before the local 50% first-freeze date. City pages carry the live widget that flags early-freeze years and the full winterizing sequence.
| City | Window opens | Close by | First freeze (50%) |
|---|---|---|---|
| Alton | Oct 8 | Oct 18 | Nov 5 |
| Arlington Heights | Sep 27 | Oct 7 | Oct 26 |
| Aurora | Sep 27 | Oct 7 | Oct 23 |
| Bloomington | Sep 28 | Oct 8 | Oct 16 |
| Champaign | Oct 1 | Oct 11 | Oct 24 |
| Chicago | Sep 29 | Oct 9 | Nov 5 |
| Cicero | Oct 2 | Oct 12 | Oct 31 |
| Decatur | Oct 2 | Oct 12 | Oct 24 |
| Elgin | Sep 24 | Oct 4 | Oct 18 |
| Evanston | Sep 25 | Oct 5 | Oct 21 |
| Joliet | Sep 27 | Oct 7 | Oct 25 |
| Naperville | Sep 24 | Oct 4 | Oct 16 |
| Peoria | Oct 1 | Oct 11 | Oct 23 |
| Rockford | Sep 24 | Oct 4 | Oct 17 |
| Round Lake Beach | Sep 22 | Oct 2 | Oct 23 |
| Schaumburg | Sep 24 | Oct 4 | Oct 13 |
| Springfield | Oct 1 | Oct 11 | Oct 22 |
| Waukegan | Sep 23 | Oct 3 | Oct 19 |
Dates are typical-year model outputs, not forecasts — each city guide carries the live widget that tracks the current year against them.
How to use these dates
Each "close by" deadline is the model's two-clock compromise for that city: ten days after its 7-day mean temperature falls back through 61°F (water goes algae-dormant soon after), but never later than a week before its 50% first-freeze date from the NOAA 1991–2020 normals. Closing inside the window seals cold, stable water under the cover; closing early seals a warm algae incubator instead.
Treat the dates as typical-year guidance, not forecasts. An early cold snap moves the real deadline — each city guide runs a live water-temperature widget that flags a 32°F night inside the 10-day forecast as urgent, and carries the full winterizing checklist in working order.