A bicentennial is 200 years. I’m assuming that is how the show is tipping its hat to DOOL’s real 50 year anniversary. I dunno why they picked a bicentennial – but I guess they couldn’t say Salem was 50 years old since the show started with the Horton’s already in place.
a Semisentenniel. Also used, although a less recognized word, is Quinquagenary. I learned that when our church had its 50th anniversary. Some of the people wanted to call it a Semisentenniel and others a Quinquagenary. The Semisentenniel won out.