"period between Ash Wednesday and Easter," late 14c., short for Lenten (n.) "the forty days of fasting before Easter" in the Christian calendar (early 12c.), from Old English lencten "springtime, spring," the season, also "the fast of Lent," from West Germanic *langitinaz "long-days," or "lengthening of the day" (source also of Old Saxon lentin, Middle Dutch lenten, Old High German lengizin manoth). This prehistoric compound probably refers to increasing daylight in spring and is reconstructed to be from *langaz "long" (source of long (adj.)) + *tina- "day" (compare Gothic sin-teins "daily"), which is cognate with Old Church Slavonic dini, Lithuanian diena, Latin dies "day" (from PIE root *dyeu- "to shine").
Compare similar form evolution in Dutch lente (Middle Dutch lentin), German Lenz (Old High German lengizin) "spring." But the Church sense is peculiar to English. The -en in Lenten (n.) was perhaps mistaken for an affix.