Mezquita cordoba

It depends on time of year and time of day. For the 4 or 5 times we’ve been there we never purchased ahead of time since we didn’t know which day we’d be there or it was off season. Some will say buy in advance if you know when you’ll be there. During tourist season, about mid-May through mid-September, it’s probably a good idea to buy in advance since it gets very crowded. Its website is: https://mezquita-catedraldecordoba.es/en/

We did not and had no problem getting in the line moved fast.
It is a place of worship first and you can enter for free between 8 am and 9:30 am

I don't believe I've ever read about a sell-out at the Mezquita, but you never know about ticket lines. If you arrived right behind a couple of 30-person tour groups, it could take a while to buy your ticket. In the absence of information to the contrary, I'd assume getting the ticket one day ahead of time would be fine. (I see tickets available for nearly all scheduled entry times over the next week or so.) What's important to keep in mind is that the site closes for several hours leading up to 4 PM on some days. It looks as if the last entry time prior to the break--on days when there is one--is 1 PM with the site nominally closing at 2 PM. I don't know whether that would provide adequate time for a visitor. European tourist sites often start shooing people out way ahead of their official closing times, so I would not assume a 1 PM entry time would provide an hour of sightseeing time.

Check the calendar for the day(s) you'll be in Cordoba to see what your options will probably be, then continue to monitor the situation. I don't know whether there's any risk that hours could be reduced on short notice because of a special event.

Both the Cathedral and the Alcazar in Seville have ticket lines you do not want to stand in. I wouldn't be surprised if Alcazar tickets sometimes sell out a day or so ahead of time, but I don't remember what sort of warning Rick provides in his guide book. He does give a good work-around for the Cathedral ticket line.

Thank you for the help!

When I went to the Mezquita with no ticket I was dismayed at the long line to buy tickets from the window. Then I noticed several ticket-selling machines right next to it, with no lines, and easily bought my ticket from one of them. This was a Saturday afternoon in early November. I doubt if you'll have a problem, especially if you can handle the machine transaction, but acraven's advice to check ahead of time is good.

Things may have changed with time plus COVID, but when I was in Cordoba a few years ago, there was free entry into the Mesquita from opening (8 or 8:30 am) until Mass at 9:30 am. BUT at 9:30 everyone needed to leave the building, unless they were worshipping at the Mass.
By the way, I had a wonderful stay in the area near the Mesquita at “Mi Casa in Cordoba”, once a large residence but then a 3-apartment inn, apartments of different sizes, mine was the smallest, with bedroom, bathroom, and a living room with a wall kitchen. Beautifully decorated with ceramics. Booked thru booking.com.
Cordoba is lovely, lots to see and do.

Last week I bought tickets the day before and all entry times were open. I think there were about 15 people in line buying tickets when we entered.