The meaning of DAY is the time of light between one night and the next. How to use day 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. Learn what day means with clear definitions, pronunciation, synonyms, and real-world examples. Simple explanations to help you use day correctly. DAY definition: the interval of light between two successive nights; the time between sunrise and sunset. See examples of day used in a sentence. The simple definition of DAY is a period of 24 hours beginning at midnight : one of the seven time periods that make up a week. See today's exact date and time updated live in every format — ISO 8601, Unix timestamp, Julian date, day of year, week number, moon phase, and 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. 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. Find out today's date, current time, day of the week, week number, and more. Your complete date and time resource.