Every dog has his day.

It means that all of us,
no matter how humble,
will have a time of prosperity, success, or glory at some time in our lives.

The first known use in English is in 1545 (in the form "A dogge hath a day") in a translation of a collection of "adages" or proverbs by the great Dutch humanist philosopher Erasmus that had been published in Latin in 1508. Erasmus didn't necessarily invent it himself; it may have been a traditional Dutch proverb. It obviously caught on - Erasmus was greatly admired and his books were much read at the time - because the future Queen Elizabeth I used it in exactly that form in a letter around 1550. About 50 years later Shakespeare used it in "Hamlet" in this form: "Let Hercules himself do what he may, The cat will mew and dog will have his day."