Semicentennials are the 50th anniversaries of events or things.
Semicentennials are usually celebrated or remembered in a special way.
Examples:
We celebrated the semicentennial of the school's founding.
The town will have a big parade for the semicentennial of its independence.
The semicentennial of the moon landing is a big deal.
The semicentennial of the library is a good reason to have a party.
The semicentennial of the invention of the telephone is a very important event.
The semicentennial of the first person in space is a day to remember.