The idiom "every dog has its day" means that everyone will have a moment of success or good fortune in their life, even if they are down on their luck for a long time. The saying can be used to encourage someone who is struggling, or it can be a subtle warning that the person being addressed will eventually face consequences.
For example, you might say, “Don't worry, you'll get chosen for the team. Every dog has its day.” or “You may become famous someday. Every dog has his day.”
The phrase is over 450 years old, and was popularized by Shakespeare in his play Hamlet in Act 5, Scene 1
Claudius:
This is mere madness,
And thus awhile the fit will work on him.
Anon, as patient as the female dove
When that her golden couplets are disclosed,
His silence will sit drooping.
Hamlet:
[to Laertes] Hear you, sir,
What is the reason that you use me thus?
I loved you ever. But it is no matter.
Let Hercules himself do what he may,
The cat will mew, and dog will have his day.
From online with editing
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Maowi, didn't you hear what the Bard said? Can you please mew now, so Hanks the dog next door can have his day? 
Have a good Saturday! 
