Strike while the iron is hot
Act decisively and take your opportunities when they arise.
This old proverb clearly alludes to the imagery of the blacksmith or farrier at his forge. If he delays in shaping the iron when it is hot a pliable the metal soon cools and hardens and the opportunity is lost.
The expression is recorded in Richard Edwards', The excellent comedie of two the moste faithfullest freendes, Damon and Pithias, circa 1566
I haue plied the Haruest, and stroke when the Yron was hotte.
See also: the List of Proverbs.
See also: Carpe diem.