Is it ensure, insure, or assure?
late 14c., short for Lenten (n.) "forty days before Easter" (early 12c.), from Old English lencten "springtime, spring," the season, also "the fast of Lent," from West Germanic *langa-tinaz "long-days" (cf. Old Saxon lentin, Middle Dutch lenten, Old High German lengizin manoth), from *lanngaz (root of Old English lang "long;" see long (adj.)) + *tina-, a root meaning "day" (cf. Gothic sin-teins "daily"), cognate with Old Church Slavonic dini, Lithuanian diena, Latin dies "day" (see diurnal).
the compound probably refers to the increasing daylight. Cf. similar form evolution in Dutch lente (Middle Dutch lentin), German Lenz (Old High German lengizin) "spring." Church sense of "period between Ash Wednesday and Easter" is peculiar to English.