In Qatar, at 3 p.m. local time (= 1 p.m. CET/7 a.m. ET). Magnus Carlsen will be playing, and likewise Sergey Karjakin, but the world's #2, 3, and 4 players - Fabiano Caruana, Vladimir Kramnik, and Wesley So won't be, alas. Still, the field will be ridiculously strong, and it should be a very entertaining event.
The tournament website is here, and a Chess24 preview article is here.