1
general::
Take advantage of favorable circumstances, as in Car sales have finally improved so we're making hay while the sun shines. This expression alludes to optimum dry weather for cutting grass. [Early 1500s]
American Heritage Idioms
2
general::
Phrase(s): Make hay while the sun shines.
Prov. If you have an opportunity to do something, do it before the opportunity expires. • Jane: While my husband’s out of town, I’m going to watch all the movies he wouldn’t take me to see. Jane: Why not? Make hay while the sun shines.
McGrawhill's American Idioms And Phrasal Verbs