The Demisesquicentennial reference article from the English Wikipedia on 24-Apr-2004
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Demisesquicentennial

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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.