Day

Day (Heb: יוֹם ) was the name God gave to the light He spoke into existence in the creation week. But in that very verese it acquired an extended meaning for a full rotation period of the earth. One chapter later, in speaking of the whole creation week, the word is again used to designate all six days.

The day was designed as a period for work, in which energy is extended in fulfilling the command of God to Adam: "to work and keep" the garden. This principle became part of the fourth Commandment which established the weekly day of rest. Much later, Jesus would confirm that this period lasted twelve hours and that it was to used for work.

Rotation of the Earth
In the first usage of the word "yom" (יוֹם) it indicates six periods in which God worked to create the universe. The use of "evening" and "morning" indicates that the "field" where God was working was moving from work to rest as it turned away from the light of His presence. This was from the west towards the east. In this way, every part of the earth received half the light needed each rotation. However, since each rotation was all in front of God, there was no daily rest from creating for the six days.

Daylight
The word יוֹם was used to name the light, or more correctly the lit part of a rotational day. This was in relation to the unlit, or dark part of the day which God called "Night". The formula in creation account is literally, "There was evening and there was morning, day # ". For three rotations the early earth depended upon a localized source of light which was replaced on the fourth rotation when the sun was placed relative to the moon so as to become the "ruler" of the day.

Definite Period
By extension, a day can be a period of time with set boundaries. The first usage in this way is in the introduction to the section set apart for the account of how God made mankind. It was in the same period of the whole work of creation that God made mankind. In that telling, Adam is told not to eat of the Tree of the Knowledge of Good and Evil, for he would experience death "in the day" that he did.

The Very Same Day
This phrase "on the selfsame day" (Heb: בְּעֶ֨צֶם הַיֹּ֤ום) is used to distinguish a particular day apart from any other day. For instance, when a command was made, it was to be obeyed that day, without hesitation. It is also used in a historical record to mark an anniversary of a day in the past.

The Third Day
In counting of days is inclusive of the starting day. This is clear in the creation account, but depends on the starting point. The narrative may implicitly mention all three days, but often assumes the intervening day.