Old English hærfest "autumn," as one of the four seasons, "period between August and November," from Proto-Germanic *harbitas (source also of Old Saxon hervist, Old Frisian and Dutch herfst, German Herbst "autumn," Old Norse haust "harvest"), from PIE root *kerp- "to gather, pluck, harvest."
In Old English with only implied reference to the gathering of crops. The borrowing of autumn and the use of fall (n.) in a seasonal sense gradually focused the meaning of harvest to "the time of gathering crops" (mid-13c.), also to the action itself and the product of the action (after c. 1300), which became its main senses from 14c. Figurative use by 1530s. As an adjective from late 14c. Harvest home (1570s) was a festive celebration of the bringing home the last of the harvest; harvest moon (1704) is that which is full within a fortnight of the autumnal equinox.