Why is the ninth month called September?
late 13c., "command on oath," from Old French conjurer "invoke, conjure" (12c.), from Latin coniurare "to swear together; conspire," from com- "together" (see com-) + iurare "to swear" (see jury (n.)). Magical sense is c.1300, for "constraining by spell" a demon to do one's bidding. Related: Conjured; conjuring. Phrase conjure up "cause to appear in the mind" (as if by magic) attested from 1580s.