1590s, "call to remembrance," from Latin commemoratus, past participle of commemorare "bring to remembrance," from com-, here perhaps an intensive prefix (see com-), + memorare "to remind," from memor "mindful of" (from PIE root *(s)mer- (1) "to remember").
Meaning "perpetuate the memory of" (by solemn act, etc.) is from 1630s. Of things, "to serve as a memento of, to perpetuate the memory of," 1766. Related: Commemorated; commemorates; commemorating.