Why is the ninth month called September?
early 14c., from en- (1) + close, and partially from Old French enclos, past participle of enclore.
Specific sense of "to fence in waste or common ground" for the purpose of cultivation or to give it to private owners, is from c.1500. Meaning "place a document with a letter for transmission" is from 1707. Related: Enclosed; enclosing.