Merry as the day is long means to be full of life, happy, carefree, lively, and full of fun.
The phrase ''merry as the day is long'' means jubilant, full of fun, enthusiastic.
William Shakespeare is the person who first said merry as the day is long.
If someone says you're merry gas the day is long they mean you're very happy and are having a very good time.
For example.
The kids were merry as the day is long at the birthday party.
He was merry as the day is long when he won the lottery.