Demisesquicentennial

Demisesquicentennial is a celebration of 75 years. The word is from demi- meaning one-half, sesqui- meaning one and a half-, from cent- meaning one hundred, and from -ennial meaning yearly.

Thus, Demisequicentennial means one half of one-and-a-half centuries, or one half of 150 years, which is 75 years.