Between March 1st and May 1st 2021.
From March 1st 2021, the Dutch tax authorities will have filled in most of your electronic tax return. All you need to do is check everything and make any changes as required.
Your tax return must be filed with the Dutch tax authorities before May 1st 2021.
For more information, please visit the website of the Dutch tax authorities  (in Dutch).
Well prepared to do your tax return