Tomorrow is another day

Well mother to morrow is a new day.
[c 1527 J. Rastell Calisto & Melebea C1^V]

A letter‥beeing delivered him‥at supper, he deferred the opening of it, pronouncing this by-word. To morrow is a new day.
[1603 J. Florio tr. Montaigne's Essays ii. iv.]

We will say no more of it at present. ‥To-morrow is a new day.
[1824 Scott St. Ronan's Well III. vii.]

Go to it, you Mag and Lonie! To-morrow's another day, and you'll need all you can hold.
[1927 P. Green Field God i. 148]

‘Maybe tomorrow,’ Faith said, trying to turn the corners of her mouth up. ‘Tomorrow's another day.’
[2001 K. Hall page Body in Moonlight vii. 127]

Related to: future

Bibliography of major proverb collections and works cited from modern editions is available here.

One may not accomplish everything today but will have another chance. For example, We've stuffed hundreds of envelopes and still aren't done, but tomorrow is another day. This comforting maxim was first put as Tomorrow is a new day about 1520, was widely repeated, and changed to its present form in the mid-1800s.