same day tickets to visit the Hidden little catholic church in Amsterdan

do you mean "The Church in the Attic"? Or somewhere else?

I think what’s confusing Nigel, is your use of the word “little” in reference to the Church in the Attic. If it’s indeed the Church in the attic you’re talking about, it’s anything but little. It’s was also never actually hidden. The authorities at the time always knew it was there, but didn’t act against it as long as the church wasn’t too obvious. So no bells ringing etc etc.
As you can see on their website, none of the time slots for today are sold out. This means that you can definitely walk up and buy tickets on the spot.
https://opsolder.nl/en/museum-our-lord-in-the-attic/

Yes, I mean the church in the attic! I'm sorry about my mistake.
Thanks for the reply.
Blanca

We went in April and we just turned up and bought tickets on the day- no need to book in advance.

We also bought tickets when we arrived (May 2023). Other people were there, but it was hardly crowded.

Same with me, early May, 2024, just showed up bought tickets, and right in. Be warned, the place could just as well be named "the Museum of Dutch Staircases" featuring the tiny, winding, staircases found in many canal side houses. There are some accommodations for access, but only to limited areas of the museum.