Hello,
The dates are not yet published, but we have made some estimations and guesses in the following article:
They will most likely open around November 17th, maybe one week earlier, but if you book a flight for one week later (that is for the weekend of November 24th), then you will for sure be able to enjoy the Christmas markets in Budapest. If you want to be even more sure, then they will guaranteed be open in all of December, unless we suddenly get a COVID-19 version 2 forcing everything to close down.