The Hebrews considered the day to start at sundown of a day and continue to the next sundown. This has roots in Genesis in the creation story.

The Hebrew words for evening and morning are sometimes associated with a decrease of entropy (a measure of disorder) of the universe.

• God created the universe and everything in it as a starting point.
• God improved the universe each day of the creation.

In computer science, a "declarative" model is where you indicate what you want done without explicitly programming it, such as in a word processor, spreadsheet, etc.

In Genesis, God provides a "declarative" model of creation.

More: Declarative models

The Second Law of Thermodynamics (heat and energy) makes a connection between increase in entropy (disorder) and the progression of what we experience as time.

More: The beginning and end of time

The German word "Morgan" ≈ "morning" where the hard Germanic "g" turned to a softer English "y" and then disappeared as the English word "morning".

• The Middle English word "morwetid" ≈ "morning".
• The Middle English word "mornynge" ≈ "morning".
• The Old English word "morgen" ≈ "morning".
• The Gothic word "maurgin" ≈ "morning".

More: Germanic g to English y

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The modern Greek word "καλησπέρα" (ka-lee-SPEH-ra) ≈ "good evening" comes from Greek words for "good" and "evening" the ancient Greek word "ἕσπερος" ≈ "evening".

The English word "vespers" is from the PIE (Proto Indo-European) root "*WEKSPEROS" ≈ "evening" as in "stretching towards the evening".

The ancient Greek word "ἕσπερος" ≈ "evening" which has a connotation of "west" is in the setting sun.

The modern Greek word "εσπέρα" (eh-SPEH-ra) ≈ "evening".

Hercules in his 11th labor had to obtain the Apples of Esperides or Hesperides, guarded by a dragon named Ladon and the nymphs called the Hesperides.

Hesperus was related to Venus as the "evening star".

The Latin word "vesper" ≈ "evening" and is the source of the English word "vesper" is a service in the evening rather than the morning.

• The French word "ouest" ≈ "west".
• The Spanish word "oeste" ≈ "west"
• The Portuguese word "oeste" ≈ "west"

The English word "west" is related to "evening" as in "vespers".

• The Middle English word "west" ≈ "west".
• The Old English word "west" ≈ "west".
• The German word "West" ≈ "west".

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The Old Church Slavonic word "вечеръ" ≈ "evening". The Russian word "вечер" (ve-sher) ≈ "evening".

The Hebrew word "יום" (yom) ≈ "day". As in:

• The six days of creation (a declarative model)
• The seventh day of rest.
• The Day of the Lord
• The Day of Atonement

The modern Greek word "ημέρα" (ee-MEH-ra) ≈ "day" is the word for day.

The Greeks and Romans considered a day to start at midnight and continue until the next midnight.

Germanic cultures used the Hebrew tradition. Though we use the Greek and Roman tradition for the beginning and end of a "day", we still use that Germanic and Hebrew tradition for some holidays.

This connection of Germanic cultures to the Hebrew tradition goes deeper than just when a day starts. There is a field of study of these connections.

One example. All the PIE language groups such as Latin (Italian, French, Spanish, etc.), Greek, Slavic (Russian, Polish, etc.), Indian (Sanskrit, Hindi, etc.) change the inflection of verbs, adjectives, etc., by changing the ending.

Germanic languages (German, English, etc.), however, often change the verb tense by changing the vowel pronunciation such as sing, sang, sung, ring, rang, rung, etc. This is similar to what is done in Semitic languages (Hebrew, Arabic, etc.).

Details are left as a future topic.

So Christmas starts the evening before, the "evening of Christmas" or "Christmas Eve". The New Year starts the evening before, the "evening of the New Year" or "New Year's Eve".

The Greek and Roman time of day went from midnight to midnight. The Hebrew (and ancient Germanic) time of day went from evening until morning.

A Christmas hymn about midnight is "It came upon a midnight clear".

Details are left as a future topic.