The switching window is one of the few fixed dates in the Dutch calendar that applies to every resident, and it passes quickly between mid-November and 31 December. Since basic cover is legally identical everywhere while everything bolted onto it is not, the things that quietly change from year to year are easy to overlook.
Every Dutch resident can switch basic health insurers once a year, in the window that opens around mid-November and runs through 31 December at 23:59 — your new policy then starts 1 January regardless of which day in that window you actually sign up. If you cancel your current policy yourself before the new year rather than just accepting the auto-switch, some insurers give you until 31 January to pick a new one, so there's a bit more slack if you're still comparing offers. What actually changes each year and is worth checking before you commit: the monthly premium, which varies a lot between insurers even though the basisverzekering (basic package) itself is legally standardised; which aanvullende verzekering (supplementary) add-ons you're bundling in, and whether last year's dental, physio or alternative-medicine coverage still applies at the same level; your preferred pharmacy or hospital's contract status with that insurer, since uncontracted care gets reimbursed less; and your voluntary eigen risico (deductible) setting, which doesn't carry over automatically. If you switch, your old insurance cancels automatically as part of the process — no separate termination needed. Compare using an independent comparison site rather than a single insurer's own numbers, since basic coverage is identical everywhere and premium is often the deciding factor.