DayA day is the time of sunlight between sunrise and sunset as opposed to night which is the period of darkness between sunset and and sunrise. This is perhaps the preliminary concept of the human civilisation.
However, when we count days we include both day and night as one day. It is by counting through this method we count seven days of the week, thirthy days of the month, and 365 days of the year. This is the general theory despite the fact that a month might consists of 28 to 31 number of days and a year may consists of 365 or 366 days.
Astronomically speaking, we count days as mean solar day. It is a time equal to 24 hours representing the average length of the period during which the earth makes one rotation on its axis.
A day is considered to begin at midnight at 00:00:00 hours and ends 24:00:00 hours.
However, in common parlance, a day is also counted between a sunrise and the next sunrise.
Nomenclature for different parts of the day
Early hours of (date) 0000 – 0400 hrs,
Early morning 0400 – 0600 hrs.
Morning 0400 – 0800 hrs.
Forenoon 0800 – 1200 hrs.
Around noon 1100 – 1300 hrs.
Afternoon 1200 – 1600 hrs.
Evening 1600 – 2000 hrs.
Night 2000 – 2400 hrs.