make hay while the sun shines

To make hay while the sun shines means to take advantage of having an opportunity to do something before you lose the opportunity. The phrase originates from the fact that you need full sunshine while making hay, therefore when you have the opportunity (such as the sun is shining) you should do it. The concept of this idiom is that if you dont do it while you have the opportunity to do it, you may lose the chance to do it at all.