Why is the ninth month called September?
early 14c., from Old French glener (Modern French glaner) "to glean," from Late Latin glennare "make a collection," perhaps from Gaulish (cf. Old Irish do-glinn "he collects, gathers," Celt. glan "clean, pure"). Figurative sense was earlier in English than the literal one of "gather grain left by the reapers" (late 14c.). Related: Gleaned; gleaning.
The corners of fields were not to be reaped, and the sheaf accidentally left behind was not to be fetched away, according to the law of Moses (Lev. 19:9; 23:22; Deut. 24:21). They were to be left for the poor to glean. Similar laws were given regarding vineyards and oliveyards. (Comp. Ruth 2:2.)