The Flash Will Be Crossing Over From the CW to CBS to Team Up With Super Girl

CBS confirmed the crossover in a press release with this information:

The Flash is speeding to National City! THE FLASH star Grant Gustin, in his role of Barry Allen/The Flash, will cross over to SUPERGIRL in an episode to be broadcast Monday, March 28 (8:00-9:00 PM, ET/PT) on the CBS Television Network. Details of the episode will be announced at a later date.