Every dog has its day

Meaning

Every dog, and by implication every person, has a period of power or influence.

Origin

This phrase is recorded as being first uttered by no less a notable as Queen Elizabeth I. As Princess Elizabeth, in a letter to her brother and in response to his request for a picture of her, she wrote:

Notwithstanding, as a dog hath a day, so may I perchance have time to declare it in deeds.

This phrase is a rarity - it was first uttered by a queen.
The letter was published by John Strype in Ecclesiastical Memorials, 1550. It appears that Elizabeth was merely quoting what was in her day already a well-known proverb, although no record of it has been found that predates her writing it down.

John Heywood recorded the proverb in the 1562 edition of Proverbs and Epigrams and Shakespeare used it in Hamlet, 1603: 

Let Hercules himself do what he may,
The cat will mew and dog will have his day.