Sesquicentennials are the 150th anniversaries of events or institutions.
Sesquicentennials are special because they mark a long time since something important happened.
Examples:
We celebrated the sesquicentennial of our town's founding.
The sesquicentennial of the school was a big event.
The sesquicentennial of the country's independence was a huge celebration.
The sesquicentennial cake was really big.
The sesquicentennial fireworks were the best ever.
We had a big party for the sesquicentennial.