The Pantheon.

I do know you don’t need one for the weekdays, only weekends. Since we weren’t there on the weekend, I can’t help you with that.

Only ever walked in. I know you didn't ask, but the last time I was there at 8.30 when they had just opened and it is lovely, so uncrowded - I have been later in the day and it gets very crowded and noisy.

We just walked in Jan 2020. Checking the website it looks like weekdays are open. Have walked in every time we visited.

In October 2021 we were there on a Saturday and we did need to make a reservation. Didn’t realize it was necessary until standing in line but were able to get a time within an hour on the website.

This is the website for the Pantheon:

https://www.pantheonroma.com/home/

This is the page where it talks about weekend or holiday reservations:

https://www.pantheonroma.com/2020/07/15/covid-19-general-info/

I was just there, here is my advice:

We lined up about 8:45, they open at 9, already a long line. Once 9 came, it was fairly quick to get in. My advice is actually just show up around 9:15, 9:30, and you'll walk right in, plus all those people who lined up before will have already left, people don't stick around long there. Download RS audio too for this place, it makes the visit better. I went on a weekday, no ticket needed, free entry.

I have a nice memory of enjoying a coffee granita (iced coffee slushy) after our visit to the Pantheon on a hot summer day. Perhaps it was a RS recommendation? Enjoy your visit!

We went on a Friday earlier this month and just stood in line mid-afternoon. The line moved really quickly - only took a few minutes before we were inside.

I believe weekends is when you need a reservation and that can be done on the official website.