Nehemiah 8:10 says: Then he said unto them, Go your way, eat the fat, and drink the sweet, and send portions unto them for whom nothing is prepared: for this day is holy unto our LORD: neither be ye sorry; for the joy of the LORD is your strength.
(KJV).
The phrase is from Nehemiah 8, in the account of Ezra reading the law to the people gathered in Jerusalem. Read it in context at Nehemiah 8:10, or the full chapter at Nehemiah 8.