once a month

每月一次

on the sly, on the spot, on the stroke, on the table, on the trail, on the trail of sb., on the turn, on the verge of, on the whole, on time, on to, on top of, on top on, on trust, on view, on-line, on-stage, on-the-spot, once, once (and )for all, once and for all, once for all, once in a while, once more, once upon a time, oncologist, oncoming, one, one after another, one another, one day, one p (also 1p), one's bread is buttered, one's opposite number, one-, one-armed bandit, one-horse, one-liner, one-man band, one-man show,