The meaning and origin of the expression: As merry as the day is long

As merry as the day is long

What's the origin of the phrase 'As merry as the day is long'?

From Shakespeare's Much Ado About Nothing, 1599.

BEATRICE:
No, but to the gate; and there will the devil meet me, like an old cuckold, with horns on his head, and say 'Get you to heaven, Beatrice, get you to heaven; here's no place for you maids:' so deliver I up my apes, and away to Saint Peter for the heavens; he shows me where the bachelors sit, and there live we as merry as the day is long.

Shakespeare also used the phrase in King John, 1595:

ARTHUR:
Mercy on me!
Methinks no body should be sad but I:
Yet, I remember, when I was in France,
Young gentlemen would be as sad as night,
Only for wantonness. By my christendom,
So I were out of prison and kept sheep,
I should be as merry as the day is long;
And so I would be here, but that I doubt
My uncle practises more harm to me:
He is afraid of me and I of him:
Is it my fault that I was Geffrey's son?
No, indeed, is't not; and I would to heaven
I were your son, so you would love me, Hubert.