Creative Assembly has almost without fail released main game trailers (not Saga titles) on Wednesdays.
Napoleon: Total War Aug 19, 2009 A Wednesday.
Total War: ROME II – First gameplay footage Sep 26, 2012 A Wednesday.
Total War: Warhammer Apr 22, 2015 A Wednesday.
Total War: WARHAMMER 2 Mar 31, 2017 A Wednesday.
Total War: THREE KINGDOMS Jan 10, 2018 A Wednesday.
This would also explain why today's teaser had two constellations for the last two Chaos Gods not teased instead of just one.