The word day has a number of meanings, depending on the context it is used such as of astronomy, physics, and various calendar systems. As a term in physics and astronomy it is approximately the period during which the Earth completes one rotation around its axis, which takes about 24 hours.

What do we mean by a day?

A day is a unit of time. In common usage, it is an interval equal to 24 hours. It also can mean the consecutive period of time during which the Sun is above the horizon of a location, also known as daytime.

How do you use to the day?

: to exactly a specified number of years It's been 100 years to the day since their great discovery. Soon after their wedding, almost a year to the day, they got divorced.