viidestoista päivä

Finnish edit

Used other than figuratively or idiomatically: see viidestoista päivä ("fifteenth day")
(idiomatic) Opposite of "now" or "at once", used as fortifier when telling someone to do something right now.
Tarvitsen sen heti, enkä viidestoista päivä.
I need it at once (and not on the fifteenth day).