If you’re in Colorado you’re automatically registered. https://www.sos.state.co.us/pubs/newsRoom/pressReleases/2021/PR20210629VoterRegistration.html

It’s the only state (it was at the time of this article at least) that has true automatic registration.

Of course republicans have been trying as hard as they can to get rid of it.

https://coloradonewsline.com/2023/03/20/how-a-red-state-exodus-from-an-interstate-voter-database-could-impact-colorado-voter-rolls/

The day itself is more just to remind people to register to vote. Registering to vote can be done any day of the year, though there is a deadline if you plan to vote in a certain election. You don't need to register more than once in a particular state, though you do need to update your registration if you move to a new address.

The deadline for registration varies by state. For my state, it's 22 days before an election. But, at least in my state (this isn't the case in every state, but it's getting there), even if you don't register by the deadline, you can still fill out a voter registration form and vote using a provisional ballot (meaning it will be counted later that week after your voter registration has processed) on election day.

Basically, National Voter Registration Day is used to avoid a lot of headaches for everyone involved.