Visiting St. Denis Basilica in Paris

It isn't required to buy tickets in advance, but recommended. However, I would guess that in late October the crypt will not be overwhelmed with tourists. Also, the church is free to enter, just the crypt needs tickets.

We were there September 26 and had a museum pass rather than tickets but there were no lines. We were able to get right in.

We visited in April with our daughter because Notre Dame was closed to tourists due to the death of the Pope -- there was a short line to get in. You might get tickets on line that morning to make it more convenient b ut it isn't necessary.
I think it is the most underrated unvisited major site in Paris (well just over the border). Here is our visit 15 years ago -- they have added railings that make visiting the tombs less interesting but it is still great.
https://janettravels.wordpress.com/2010/10/26/confronting-mortality-at-st-denis/
After we visited the basilica we got lunch on the square and then walked to the Legion of Honor park -- it is next to the church but there was no entrance convenient that we could find so it was a longish walk. Beautiful park. We then ended up finding a different metro back to Paris from the Basilica St. Denis stop we had arrived on.

Thank you, friends! It’s nice to know there is a community of fellow travelers out there, always willing to help and provide advice.

If you're looking for lunch in the area, I recommend O' Grand Breton, affordable and pretty good! You can book online.

Always keep your head at St. Denis!

Jenny,
I don't know about your Paris timeline, but I found a good way to bookend some Paris time is to go to Reims (can be done in one full day) and visit the cathedral there. Nearly every king of France had their coronation there. It's nice to see the place where their reigns began, and where they ended, at St. Denis.
(Reims also has champagne houses to visit and the museum where the surrender by the Germans after WWII was signed.)

If you want to see things about the coronations in Reims you have to go to Palais du Tau, next to the cathedral. However is closed for the moment and reopens somewhere next year. Still have to visit it as I was in Reims during the weekly closing day of the palais. So can't say how interesting it is.

With St. Denis you can’t go wrong anyway, so much history packed in one building is really impressive. And easy to get by metro.

Easy to get to by Metro. No need for advanced ticket purchase. We just showed up and bought tickets there.

To me it's a very important place to see. We took a guided tour in French (not sure if one is available in English, but totally worth checking out). Our guide was sensational.