A day in which an employee is not scheduled to work. / A day in which a person is not performing up to their usual level of ability.

(law) A day on which a court is open.

A US national holiday celebrated on the second Monday in October, for Christopher Columbus' arrival in the Americas.

A paid personal day intended to be used due to illness either for recuperation or medical treatment.

A day when one is permitted to not attend work in order to attend to personal matters.

A day in summer.

(idiomatic) A difficult period of need, when things do not go right. / Used other than figuratively or idiomatically: see rainy, day.

the next day.

(childish) A contrived holiday on which everything is done backwards, for example "yes" means "no".

For the period of an entire day, or at least during all the daytime of a day.