Every dog has its day

What's the meaning of the phrase 'Every dog has its day'?

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

What's the origin of the phrase 'Every dog has its day'?

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 pre-dates 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.