The 1 May date is well known; the exceptions around it are not. Migration years run to a later deadline, an extension can only be requested once an invitation to file has actually arrived, and interest on an outstanding balance follows its own clock regardless of any extension granted.
The standard deadline for filing your income tax return (aangifte inkomstenbelasting) is 1 May of the year after the tax year — so the 2025 return is due 1 May 2026. If you moved to or from the Netherlands during the year and need an M-form instead, that deadline is later, generally 1 July. Missing 1 May isn't automatically a problem if you act first: you can request an extension (uitstel) through Mijn Belastingdienst, by phone, or by post, but only after you've actually received an invitation to file. That typically pushes your deadline to 1 September, though interest on any tax you end up owing (belastingrente) can start accruing from partway through the year regardless of the extension, so requesting one doesn't make a balance due any cheaper. If you hire a tax adviser or intermediary, they often file under their own staggered arrangement with the Belastingdienst, which can stretch the practical deadline considerably further into the following year — one more reason a complex first-year or M-form situation is often worth handing to a professional rather than racing the clock yourself. ---