The CW announced that starting next month Supergirl and Legends of Tomorrow will alternate airing in the 8 PM Monday time slot through June. I guess this is how the CW is making room for Black Lightning.

From Deadline:

Supergirl already was set to return January 15 in the time slot and will air originals until Legends takes over on February 12. After nine weeks Supergirl will fly back into the 8 PM hour with more originals from April 16 to June 18.