c. 1300, "a memory, recollection," from Old French remembrance (11c.), from remembrer (see remember). From late 14c. as "consideration, reflection; present consciousness of a past event; store of personal experiences available to recollection, capacity to recall the past." Also late 14c. as "memento, keepsake, souvenir," and "a commemoration, remembering, ritual of commemoration." Meaning "faculty of memory, capability of remembering" is early 15c.
British Remembrance Day, the Sunday nearest Nov. 11 (originally in memory of the dead of World War I) is attested from 1921. A remembrancer (early 15c.) was a royal official of the Exchequer tasked with recording and collecting debts due to the Crown; hence also, figuratively "Death" (late 15c.).
To us old lads some thoughts come home
Who roamed a world young lads no more shall
[Melville, from "To Ned"]