Strike while the iron is still hot
Not to hesitate in doing something, especially once you have made your decision to do it. Not to waste time to take action when trying to achieve a goal, especially once you have recognized all conditions favor you.
In the 1300´s Geoffrey Chaucer used this expression after seeing that blacksmiths used to hammer red hot pieces of metal so as to shape them. Then, he extrapolated the literal use of this idiom to those situations in which a decision was to be made in the heat of the moment so that a person could take advantage of any given situation once it appeared.
-They are offering me a scholarship to study in England.
-Wow! When are you leaving?
-I don´t know if I should take it. I first want to take English classes in order to improve my level before attempting such a feat.
-What? Are you crazy? But you speak English proficiently.
-I don´t think so. I guess I can still learn more.
-You know what? if you don´t take advantage of this great opportunity now that it is there, you will regret it the rest of your life.
-Really, do you think so?
-Of course, it is now or never. You might never in your life have a chance like this again. Strike while the iron is still hot!