During Sunday night's episode of Supergirl, the first official promo of the 2018 Arrowverse Crossover premiered.
The crossover begins Sunday, Dec. 9 at 8/7c, but it won't start with Supergirl. For one week only, Supergirl and The Flash are trading time slots, so double check your DVRs! The Flash will kick off the three-part crossover on Sunday, Dec. 9, with Arrow following on Monday, Dec. 10, and Supergirl finishing things off on Tuesday, Dec. 11.
(Source)