Supergirl airs Mondays at 8pm on The CW, The Flash airs in the same timeslot on Tuesdays, Arrow on Wednesdays, and Legends of Tomorrow on Thursdays.
The CW has just released a new promo teasing the four-part crossover, which features Barry Allen introducing Supergirl to members of Team Arrow, Team Flash, and the Legends.
Not too long after Supergirl ended its freshman run on CBS, rumors began to swirl that the series would join Arrow, The Flash, and Legends of Tomorrow on The CW.
During the second season of The Flash it was established that Supergirl, and her friends in National City, existed on an Earth outside of Earth-1 and Earth-2.
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The Flash, Arrow, Supergirl, DC's Legends of Tomorrow - 4 Night Crossover Teaser Promo (HD)