The meaning of DAY is the time of light between one night and the next. How to use day in a sentence. Find out today's date, current time, day of the week, week number, and more. Your complete date and time resource. Define day. day synonyms, day pronunciation, day translation, English dictionary definition of day. n. 1. The period of light between dawn and nightfall; the interval from sunrise to sunset. 2. a. The 24-hour period during which the earth completes one. DAY definition: the interval of light between two successive nights; the time between sunrise and sunset. See examples of day used in a sentence. A day is the time period of a full rotation of the Earth with respect to the Sun. On average, this is 24 hours (86,400 seconds). As a day passes at a given location it experiences morning, afternoon, evening, and night. This daily cycle drives circadian rhythms in many organisms, which are vital to many life processes. A collection of sequential days is organized into calendars as dates. Details about today's date with count of days, weeks, and months, Sun and Moon cycles, Zodiac signs and holidays. Discover today's unique celebrations, holidays, and observances on National Today. Find out what makes this day special and worth celebrating! Discover what national day is celebrated today and explore its history, themes, and activities. DAY definition: 1. a period of 24 hours, especially from twelve o'clock one night to twelve o'clock the next night…. Learn more. 🎉 Browse hundreds of national days, holidays and celebrations. Find today's national days, explore what's coming up this week, and never miss a fun holiday again.