Legends of Tomorrow has had a pretty solid place in The CW's primetime slate on Tuesday nights following The Flash, which is generally the network's biggest ratings winner. The show will move from its post-Flash slot for a time in 2018, however, and the move means big changes to The CW's superhero lineup. Because new superhero series Black Lightning will take over the 9 p.m. Tuesday slot, Legends will move to Mondays at 8 p.m. into the slot usually held by Supergirl. While the tweaks to the lineup are obviously good news for those of us who have been waiting to see Black Lightning, they're unfortunate for fans of Supergirl.
Supergirl will be off the air for nine weeks in the first half 2018, with Legends of Tomorrow in its regular time slot and no room elsewhere in the lineup thanks to Black Lightning. Fans of the Girl of Steel's adventures in National City will have to go more than two months without any new episodes in a break that will come right in the middle of Season 3 and shortly after the winter hiatus. Kara and Co. will be back briefly after winter hiatus, with new episodes from January 15 through February 5. At that point, Legends switches to Mondays, Black Lightning comes to Tuesdays, and Supergirl takes a break.
The good news is that Supergirl hasn't gotten its episode order cut just to end at the same time as the other superhero shows presumably will in May. Viewers can expect to see Legends of Tomorrow on Monday through April 9, at which point the rest of Supergirl Season 3 will begin to air. THR reports that the rest of Season 3 will air through June 18. Supergirl will still get a full season; we'll just have to wait a little longer for it.
Interestingly, the schedule switcheroo means changes to The CW's superhero universe that go beyond which shows air on certain days of the week. For the first half of the 2017-2018 TV season, The CW made sure to keep the four superhero shows on track with each other so that the timelines would fit for the big "Crisis on Earth-X" crossover. Smaller crossovers -- such as Felicity's visit to The Flash -- were possible because the timelines matched. Now, crossovers between the other three shows and Supergirl almost certainly can't make sense. We may see characters from Arrow, The Flash, and Legends interacting; the folks from Supergirl will probably be staying on Earth-38.
The episodes of Supergirl that air prior to Legends of Tomorrow taking over will introduce the Legion of Super-heroes beyond just Mon-El and Saturn Girl. We'll have to wait and see if we'll be stuck with a cliffhanger from early February to early April. You can catch the series premiere of Black Lightning on Tuesday, January 16 at 9 p.m. ET. Black Lightning hasn't been established as taking place within the Arrow-verse, so we probably won't get any crossovers with the new series. Still, we'll have to wait and see.