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.