Semiquincentennial is a very long word for a very big anniversary — 2026 is the semiquincentennial, or 250th anniversary, of the United States.
Semiquincentennial derives from the Latin words semi, "half," quinque, "five," and centum, "one hundred." You don't need this word very often, since it's rare that a country, building, or organization marks its 250th anniversary, but when you do, it's a big deal. The British Parliament celebrated its semiquincentennial in 1957, and the Taj Mahal was built in 1648, which means its semiquincentennial happened back in 1898.