Make hay while the sun shines.

Take advantage of a favorable opportunity while the conditions that make success possible are still present. Delay can waste a chance that may not return once circumstances change.

The image comes from work that depends on dry, sunny weather: hay must be prepared when the weather allows it. The saying therefore encourages timely, practical action rather than passive waiting.

Used to urge someone to act promptly when conditions are especially favorable, such as during a brief business opening, a period of available resources, or a chance to complete work efficiently. It can sound encouraging or gently pressuring.