Day n. : The time of light, or interval between one night and the next; the time between sunrise and sunset, or from dawn to darkness; hence, the light; sunshine.

Day n. : The period of the earth's revolution on its axis. -- ordinarily divided into twenty-four hours. It is measured by the interval between two successive transits of a celestial body over the same meridian, and takes a specific name from that of the body. Thus, if this is the sun, the day (the interval between two successive transits of the sun's center over the same meridian) is called a solar day; if it is a star, a sidereal day; if it is the moon, a lunar day. See Civil day, Sidereal day, below.

Day n. : Those hours, or the daily recurring period, allotted by usage or law for work.

Day n. : A specified time or period; time, considered with reference to the existence or prominence of a person or thing; age; time.

Day n. : (Preceded by the) Some day in particular, as some day of contest, some anniversary, etc.

Example:
How many calories a Day should I consume while dieting?

Taxonomy
landings, D-Day, beaches, troops, invasion, German, British, beachhead, 6 June 1944, the Germans, Omaha Beach, infantry, Normandy, 1944, attack, objectives, Sword Beach, Utah Beach.

Defination from the web:
day of actual landing, which was dependent on final approval. ... kilometres (5 mi) by the end of the day but failed to make some of the deliberately ... The Longest Day (film), the 1962 Oscar-winning film dramatization of the event ...