Christmas Markets in Vienna will begin opening this year on Nov. 6 - a little more than two months from now. I'm setting up this thread now where I will share my reviews and insights.
As a first observation, the Christmas Market at Belvedere will not take place this year due to renovation works. A new Christmas Market by the same management company will take place for the first time at Schwarzenbergplatz in front of the large fountain. Also notably, the Christmas Market at Maria-Theresien-Platz is back this year after a hiatus last year due to construction.
Here's a link with dates/times - https://www.wien.info/en/now-on/christmas/christmas-markets-342664