A day on which university students do silly things for charity; often the culmination of rag week.

(informal) Father's Day

(US) An alternative later date for an outdoor event in case of inclement weather.

(astronomy) The rotational period of Earth; the time in which Earth rotates exactly 360 degrees around its axis, measured relative to the stars (about 23 hours, 56 minutes, 4 seconds).

(astronomy) One day of sidereal time; the time in which Earth rotates once around its axis, relative to the point of the vernal equinox (about 23 hours, 56 minutes, 4 seconds).

(UK) Alternative form of day laborer

As frequently as every day; daily.

(astronomy, physics) A unit of length equal to the distance light travels in one day

Daybreak.

(idiomatic) To seriously regret one's actions.