monthly (adv.)

"once a month, in every month," 1530s, from month + -ly (2).

1570s, "pertaining to or relating to a month," from month + -ly (1). Old English had monaþlic, but the modern words seem to be separate formations. From 1580s as "continued for a month;" 1640s as "done or happening once every month." As a noun, "a magazine or periodical published once a month," by 1833.