Wiktionary
make hay
vb. 1 (context literally English) To cut grass to turn into hay for animal feed. 2 (context idiomatic English) To take advantage of an opportunity.
WordNet
make hay
v. turn to one's advantage; "The environmentalist lobby made hay of the nuclear plant accident"
Usage examples of "make hay".
Yet the work had just begun with the bringing of the cattle, and while the Indians rode herd, I taken Galloway, Nick, and Charlie Farnum out to make hay.
You better make hay with her during the next few days, because that's all there's going to be.
You better make hay with her during the next few days, because that’.