A day of vacation; a day when one does not go to work, school, etc.
After the accident, he took a day off.
Tuesday is my day off.
2022 December 14, David Turner, “The Edwardian Christmas getaway...”, in RAIL, number 972, page 35:
In addition to the extra labour at Christmas, because services kept running on Christmas Day many of the staff did not get the day off. […] Moreover, after Christmas Day he got a few days off and had "...a real old jolly time, as only we men of the iron road know how".