Versallies on Sunday?

I would go with Tuesday. Weekends are generally heavier with the locals attending.

Does the Sunday include the music fountain concert? If so, that would be my choice

The fountain shows end in October so you won't be there for them to be on no matter which day you choose. Compared to summer, both days are light to moderate.

I’d never go to Versailles on a weekend, always more crowded for the reason Frank said.
I’m in Paris now, rainy, cold here all day, just sat next to an American couple at a café that waited in line for 2 hrs today (Sunday) at Versailles... in the rain.

For others reading this, Wed-Thurs are the least busy days per a lovely woman who works at Versailles - and based on my experience. I love Versailles, been there 50 times?

Appreciate the input and I'll be avoiding that Sunday visit.

Thanks much!

Two factors: crowds and rain. If there's no rain on that Sunday, I'd take advantage of the weather and just go, despite the crowds. You can start with the gardens first and the palace in the afternoon. Secondhand knowledge gathered here says there's a garden entrance and the palace is less crowded in the afternoon.
Otherwise, if it's raining, wait until Tuesday. Don't count on weather predictions being accurate if you look at an app. The weather forecast for the Paris region is useless because it is so close to the Atlantic.

Actually, we went on a slightly-rainy day and the tourists "stayed away in droves" so we were able to see the mirrors in the Hall of Mirrors. On an earlier visit we only able to see the shoulders of the people around us.
Not the best for viewing the gardens, though.

Good point, Laura.
The same happened to us but it was after a downpour and at the Eiffel Tower. Personnel said the tour guides had cancelled their group visits, so there was no line. Even without a reservation, we were instantly in the elevator going up.

My experience is a little dated, as I was last there on a Sunday in early 2010. It was an absolute disaster due to crowding. Rick advises to avoid Sunday, and I should have listened to him.

Thanks so much for the addendums!

My best experiences in life have occurred during pouring rain and other crazy storms, and since I love gilt and the 17th and 18th centuries, I'm very excited to be there on the least crowded of dates.

I am now actually considering Thursday based on some of the comments here, which is the day before I fly out. I figure it is a small risk to do it the day before I leave (in case something were to happen that would prevent the visit) but really hope to avoid the crowds.

Is a reservation necessary? We will be there in November and will be using our Paris museum pass...