Demisequicentenial is a celebration of 75 years. The word is from demi- meaning one-half, sequi- meaning one and a half-, from cent- meaning one hundred, and from -ennial meaning yearly.
Thus, Demisequicentenial means one half of a one-and-a-half centuries. or one half of 150 years or 75 years.
Generally speaking, few celebrations are called Demisequicentennials, most are called silver anniversaries, after a gift giving chart for marriages.