opportunity knocks
A major opportunity to find success presents itself or becomes available. You have to be ready to act if opportunity knocks, or you might end up regretting it forever. It was hard leaving my parents and all my friends from high school, but when opportunity knocked to attend school overseas, I knew I had to seize the chance.
opportunity knocks
a chance of success occurs. This expression comes from the proverb opportunity never knocks twice at any man's door or opportunity knocks but once . The form of the saying with opportunity dates from the late 19th century, but fortune was used in the early 19th century and a version of the saying is recorded in medieval French.