From the word bissextus, which refers to February 29th.
Bissextus means literally “the second sixth,” so called because the 6th day before the first day of March in the ancient Roman calendar appeared a second time once every leap year.
Comes from the past participle of the Latin verb adumbrāre, “to shade.”
Adumbrāre has the stem umbr(a), “shade, shadow,” also found in the Astronomy terms umbra, “the complete or perfect shadow of a planet,” and penumbra, “the partial or imperfect shadow outside the complete shadow of a planet.”