day meaning
EN
[deɪ] [-eɪ] 
US UK WDay
- 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.
- Several definitions of this universal human concept are used according to context, need and convenience. In 1967, the second was redefined in terms of the wavelength of light, and it became the SI base unit of time.
- In recent decades the average length of a solar day on Earth has been about 86 400.002 seconds (24.0000006 hours) and there are about 365.2422 solar days in one mean tropical year.
EN Day 
- NounPLdays
- Any period of 24 hours.
- I've been here for two days and a bit.
- A period from midnight to the following midnight.
- The day begins at midnight.
- (astronomy) Rotational period of a planet (especially Earth).
- A day on Mars is slightly over 24 hours.
- The part of a day period which one spends at one’s job, school, etc.
- I worked two days last week.
- Part of a day period between sunrise and sunset where one enjoys daylight; daytime.
- day and night; I work at night and sleep during the day.
- A specified time or period; time, considered with reference to the existence or prominence of a person or thing; age; time.
- Every dog has its day.
- A period of contention of a day or less.
- The day belonged to the Allies.
- Any period of 24 hours.
- VerbSGdaysPRdayingPT, PPdayed
- More Examples
- Used in the Middle of Sentence
- I have always argued that it is one that deserves a thorough-going policy review, which might come to all sorts of conclusions, including it is doing what the Government of the day wants it to.
- On the third day after coil embolization, the follow up tubography showed the successful closure of fistulous tract.
- The Mets, a day after scoring 9 runs on 12 hits, managed four singles against Aaron Cook, the Rockies’ sinkerballing right-hander, who pitched a complete game.
- Used in the Beginning of Sentence
- Days after announcing his presidential bid, Senator Rand Paul, Republican of Kentucky, tried a quirky approach to skipable online video, first popularized by Geico.
- Used in the Ending of Sentence
- Six weeks into the season, even this intrepid exhibitiongoer had her days.
- Dentists recommend several toothbrushings per day.
- For example, the terrorist attack in New York on September 11, 2001, caused a sudden drop in the US market and had a flow on effect to the Australian market, which dropped about 10% within a few days.
- Used in the Middle of Sentence
Definition of day in English Dictionary
- Part-of-Speech Hierarchy
- Nouns
- Countable nouns
- Countable nouns
- Verbs
- Nouns
Source: Wiktionary
Grammatically, this word "day" is a noun, more specifically, a countable noun. It's also a verb.