A day is roughly twenty-four hours because that is how long Earth takes to rotate once.

The longer version

Rotation carries Earth's surface through sunlight and darkness once per cycle. A spot facing the Sun at noon keeps turning, passes through dusk and night, and returns to the same angle about a day later. The interval is not perfectly constant, since axial tilt and a slightly elliptical orbit make apparent solar days vary a little across the year.

Civil time smooths those variations into a fixed twenty-four hour day, so clocks run at a steady rate in every season. Before mechanical timekeepers existed, communities reckoned daylight between sunrise and sunset, which meant an hour shrank in winter and grew in summer. Equal hours later made wages, schedules and long-distance travel far easier to arrange.

Where this entry sits

These facts wander through clocks, calendars, time zones, and the long human effort to measure passing hours. You will find leap years, odd month lengths, ancient sundials, and the reasons modern timekeeping works the way it does. The full list sits on the Time and Calendars page, where every entry is listed in order.