Origin
Attested since 1546, originally a Tudor expression, and used figuratively since 1673.
Explanation
Make the most of one’s opportunities while you have the chance. If you have an opportunity to do something, do it before the opportunity expires.Hay is difficult to prepare in wet weather, so the proverb points to the wisdom of taking advantage of opportunities while they’re available. If you make hay while the sun shines, you make good use of the chance to do something while it lasts. In medieval times, when forecasting the weather several days in advance was more difficult, farmers would have to take several days to cut, dry and collect hay depending on the weather conditions. Make hay when the sun shines means the remains of rice fields after harvest are dried in the hot sun on the tar roads and then tied into bundles and stored as food for the cattle throughout the year. Drying of the grass into hay can be done only when there is hot sun; otherwise, the whole grass is wasted if it rains in a freak manner during summer.
Examples
Jane : While my husband’s out of town, I’m going to watch all the movies he wouldn’t take me to see.
Jack : Why not? Make hay while the sun shines.
I’ve got a few hours to finish the housework before the kids come home, so I might as well make, hay while the sun shines.