Today didn't go so well, but we'll have another chance tomorrow. The proverb has been traced back to about 1520 (Calisto & Meliboea by J. Rastell), and is first attested in the United States in Earl Derr Biggers's book, Seven keys to Baldpast (1931). The proverb was popularized through Margaret Mitchell's Gone With the Wind (1936) and the 1939 movie based on it. The proverb is found in varying forms : Tomorrow is a new day; Tomorrow is also a day.

1931. Tomorrow is another day. - Earl D. Biggers, seven Keys to Baldpate
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1992. But if Scarlett O'Hara had it right-if "tomorrow is another day" - then can we expect a better four years ahead if Bush is returned to office?
-- New York Times
1993. Hillary (Clinton) works tirelessly to counteract Bill's belief that tomorrow is at least another day away. - Time

1994. "I shut my eyes tight. Tomorrow is another day, but I want to sleep for a hundred years. Then, when I wake up, my father will be back. There'll be paper chains in the room and we'll be eating cake. 'In the camp,' he'll say, 'I had bread made out of sawdust.' he'll smile. he will never leave us again."
-- New York Times

I've survived another work week. Its almost over. Typically said by working people on Friday afternoon before the weekend begins. This saying is often abbreviated to TGIF, and each letter is pronounced separately.

1970. There is a vase world of work out there in this country, where at least 111 million people are employed in this country alone - many of whom are bored out of their minds. All day long. Not for nothing is their motto TGIF- " Thank God it's Friday." They live for the weekends, when they can go what they really want to do. - Richard Nelson Bolles, What Color Is Your Parachute?
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1994. Thank God It's Thursday. At many restaurants and clubs, especially in New York, Thursday night has become the big social night of the week.
- New York Times