Advance tickets needed for Mezquita?

It has been a couple years since I was there, and I just went on the app and bought them about an hour before I went. There should be decent availability, it is a large area, so I doubt they ever really "sell out".

As for timing, early or late in the day is probably safe. I sensed there were a lot of tour groups and daytrippers that get there in the middle of the day.

Same experience as Paul - except we literally were in the courtyard when I bought them through the app.

Time of day (inside) didn't seem to matter super much, but I generally feel that most things do get a bit busier as the day progresses (before getting less busy), so mid-morning seems best for us.

We also did the tower climb/viewpoint and that was a nice add-on to the whole visit.

We were there last year. Went first thing in the morning, with a timed ticket. By mid-morning the tour groups had arrived, but the place is so large that we didn't feel too crowded. The cathedral in the center can get crowded. We sat down in the cathedral and waited for various tour groups to finish and leave. But then the next collection of tour groups came in.
We were inside the Mezquita for a couple of hours. Truly a magnificent place to visit, and very easy to see everything.

We’ve always just walked up and it was never an issue, but we generally don’t travel in June.

Tour groups tend to visit sites in the morning, so either get there when it opens or go in the afternoon after they’ve gone.

You can enter for free in the morning from 8 a.m. to around 10 a.m. We did that last year and enjoyed the cooler temps, smaller crowds and watching things being set up for morning mass.