Archimedes' principle indicates that the volume of displaced fluid is equivalent to the volume of the object immersed in that fluid. Drop a bowling ball into a bathtub full of water, and it will displace an amount of water equal to that bowling ball.
To put this into television terms, you can't launch a new show without first "displacing" another.
There being only so much programming time available in a schedule, ABC opted to displace — with extreme prejudice — the daytime drama NEVER TOO YOUNG. The soap was aimed at teens, and starred Tony Dow and Dack Rambo (years before his name became unintentionally ridiculous, thank you very much Mr. Stallone.) NEVER TOO YOUNG aired its final episode Friday, June 24, 1966. When viewers tuned into the timeslot the following Monday, they found a spooky, gothic soap opera called DARK SHADOWS in its place. Happy anniversary!
This was no surprise to anyone involved with NEVER TOO YOUNG. ABC had been promoting DARK SHADOWS for some time before it premiered on June 27, 1966. And, as famously terrible as DARK SHADOWS' ratings were during the first year, they were marginally better than those earned by NEVER TOO YOUNG. Had ABC had the benefit of a more advanced ratings system in 1966 (networks discovered several years later the value of younger audiences) there's a slim chance we never would have seen DARK SHADOWS. It's difficult to say, really, because anyone who'd cancel a show with Tony Dow is a heartless monster and can't be trusted to make rational decisions.
If you're so inclined, you can watch an episode of NEVER TOO YOUNG in the video below . It appears to be the final episode of the series, broadcast on Friday, June 24, 1966 ... just days before the debut of DARK SHADOWS.