Paris Timing?

One thing to mention is that September is a busy time in Paris. The weather is lovely. September and October (and May) are my favorite months in Paris. The Saint Germain area is lovely for a first visit but if you are planning to stay a month, you may want to consider some less idealized areas of the city because you will have the time to make your way around town.

I've gone in the Sept/Oct time frame a number of times. I'm headed back to meet up with a tour in the middle of my time frame and discovered that there was a definite price drop for hotels Oct 1. I know that September is always a busy month but it looked like some hotels were still charging "Olympic" rates (insert eye roll emoji here!) and they dropped Oct 1. So...I'm heading over around the 1st of Oct for a smidgy of savings on lodging and coming back the end of Oct. Your short term rental may not be much different price wise, but it's worth checking out whether a day or 2 delay will save you some $$.

I always take a waterproof rain jacket to Europe. I've gotten rain in October but also had very pleasant Fall days. I did not go last October but I think some of the forum members mentioned last October was the wettest they've ever encountered. That might bode well for this year, lol.

Still, no matter what the weather, I love Paris in October and get plenty of walking in. You may also need a warm layer under your waterproof jacket by month's end.

Ah, first time visit. St. Germain is a beautiful area yet not necessarily convenient. If you want convenience, I suggest near Hotel de Ville on the right bank. It's near the Marais and the #1 line of the metro. Buses are also convenient in the area. Good idea to bring the rain jacket because you never know in Paris. Today, it was sunny, cloudy, pouring rain, light rain, and hail. Have a wonderful time in Paris on your 20th anniversary. It's a beautiful city, and I love it.

I hope you're using Booking.com as one of your sources for potential apartment rentals.

I also hope you will be out and about at least one evening when it's wet. One of my most treasured memories of Paris is getting a taxi near the Eiffel Tower to take us home to our apartment near the Marché Couvert des Enfants Rouges in the Marais. It was rainy and wet. The lights and their reflections on the wet pavement were atmospheric, romantic and like being in a painting or a movie.

Yeah October is normally so nice, but last October sucked weather-wise, just like this spring has.

A favourite small fall pleasantry: In Late the September the chestnut trees will be dropping their leaves, littering gravel park paths and crunching like crackers under my shoes.
An unpleasantry: The municipal gardeners appear to work to the calendar rather than the weather conditions and pull out the still-blooming ornamental flowers on schedule

We have stayed in the St Germain area many times and find it very convenient. South of Blvd St Germain and west of Jardins du Luxembourg is a bit quieter. We love taking the bus and there are many routes that include that area. One point to be aware of looking for short term rentals is that Paris limits the number of nights per year and many may have used up their 120 days already. AirBNB listings are particularly notorious these days for last minute cancellations so be aware. There are websites with apartment rentals that operate as a business all year round so they are a better option and not subject to the 120 day rule. There are also apart-hotels available.