Make hay while the sun shines
Make the most of one's opportunities while you have the chance.
This proverb is first recorded in John Heywood's A dialogue conteinyng the nomber in effect of all the prouerbes in the Englishe tongue, 1546:
Whan the sunne shinth make hay. Whiche is to say.
Take time whan time cometh, lest time steale away.
Many proverbs exist in other languages, but this one doesn't and it's a reasonable surmise that the phrase is of English Tudor origin.
Of course, medieval farmers would be as well aware of the wisdom of not leaving it too late to gather one's hay. Modern machinery and weather forecasting make haymaking reasonably quick and stress-free. Tudor farmers would have taken several days to cut, dry and gather their hay and would have had only folk rhymes like 'red sky at night' to guide them. Forecasting the weather two or three says in advance wouldn't have been possible, so all the more reason for them to 'make hay while the sun shines'.
The proverb, like all proverbs, was extended to life in general and it quickly became a cliche. As early as 1673 it was cited in a figurative, that is, non-farming, context, in Richard Head's glossary of the language of theives and beggars The Canting Academy:
She ... was resolv'd ... to make Hay whilest the Sun shin'd.
See also: the List of Proverbs.