how do i get same day pass

I went on Sunday with my family and there was no line. If you're military or Law enforcement, there is a dedicated line, as well.

As others have said, there are public Access hours during weekdays where no pass is needed. Otherwise you will have to go to https://nmaahc.si.edu/ to get passes for weekends.

A pass is no longer needed. But check the website.

Try your luck with walk-in

You will have to try going online. They have public access hours. No passes needed