Why is the The Nativity called "Christmas"?

I subscribe to the minority opinion here that the Holiday originated first in Egypt, but since Alexandria and Rome have close ties what became Egyptian soon became what was Roman.
This sort of thing has happened before on other issues, Coptic Patriarch used the term "Pope' before the Patriarch of Rome used that term etc. But it should be noted that the "mos" syllable being such a basic sound that is very much open for interpretation as it sounds like many words (especially Mass).

Pharaoh of Egypt

“And it came to pass the third day, which was PHARAOH'S BIRTHDAY, that he made a feast [BIRTHDAY PARTY] unto all his servants: and he lifted up the head of the chief butler and of the chief baker among his servants. And he restored the chief butler unto his butlership again; and he gave the cup into Pharaoh’s hand: but he hanged the chief baker: as Joseph had interpreted to them” (Genesis 40:20-22).

When pharaohs were crowned in ancient Egypt they were considered to have transformed into gods. This divine promotion made their coronation date much more important than their birth into the world. Scholars have pointed to the Bible’s reference of a Pharaoh’s birthday as the earliest known mention of a birthday celebration (around 3,000 B.C.E.), but Egyptologist Dr. James Hoffmeier believes this is referencing the subject’s coronation date, since that would have been the Pharaoh’s “birth” as a god.

This Is Why You Get To Celebrate Your Birthday Every Year

The Romans however are widely credited for the celebration of birthdays among the everyday people if you do any study on the subject (This includes the birth cakes tradition) However, I believe the Coptsic Christians are the originators based on an entry speaking about Coptic, as well as the long established Egyptian tradition of celebrating deities and royalty (Christ qualifies as both).

The origin of the word Christmas:

According to the recent researches, it is proved that the word "Christmas" is of an ancient Egyptian origin. Where this word is consisting of two parts: Christ & mas; Christ is the Lord, but what is mas?

Mas is an ancient Egyptian word means: to be born or Birth. The word is drawn in the Hieroglyphic language -the ancient Egyptian language-as follows:

Christmas in Egypt

According to dictionary.com Christmas is from the Middle English "Mass of Christ"

The Feast of the Nativity is the celebration of Jesus's birth.

It seems like it was called Christmas because it was the day where everybody went to Mass to celebrate the birth of Jesus Christ

If it was "Nativimas" it would loosely translate to "Mass of the birth of Jesus Christ"

Another reason it is Christmas instead of "Nativimas" is because Nativity is a word taken from French and Latin origin unlike Christmas, which is exclusively English. Both fully emerged around 1150 though.

Why is the Nativity called “Christmas”?

There is no clear cut source as to why the word "Christmas" became such a popular term at this time of the year than that of the "Nativity".

However there are a few hints as to why this came about. Let us take a look at the etymology of the word "Christmas" first.

“Christmas” is an Old English word, constructed from the combination of two words, namely “Christ” and “Mass”. The first recorded Old English version of the phrase, “Crīstesmæsse,” dates back to 1038, but by the Middle Ages the term had already morphed into “Cristemasse;” a slightly more modern version of the phrase.

The two separate parts of the word can be traced back to Greek, Hebrew and Latin origins. “Christ” comes from the Greek word “Khrīstos” (Χριστός) or “Crīst,” and there’s a lot of evidence to suggest that the Hebrew word “Māšîaḥ” (מָשִׁיחַ) or “Messiah,” which actually means “anointed,” has also played a considerable role in the construction of the first part of the word “Christmas.” The second part most probably comes from the Latin word, “Missa,” which refers directly to the celebration of the Eucharist. - Where does the word Christmas come from?

So, we can see that the word "Christmas" could actually mean the "Mass of the Anointed One", for that is what the word "Christ" truly implies and means in itself.

We must remember also that during the Middle Ages, Latin was more popular in the liturgical books of the day.

The fact that the English term for the Navivity of Our Lord Jesus Christ is called Christmas and not ""Nativitissimas" or something similar is due to the fact that the Church also celebrates the Nativity of Blessed Virgin Mary (September 8) and the Nativity of St. John the Baptist (June 24). There can not be three feasts called "Nativitatissimas'. That would just be too confusing to say the least.

The whole idea of calling the Nativity or Birth of our Lord Jesus Christ instead of Christmas reminds me of the solemn Proclamation of the Birth of Christ that one time was pronounced (sung/chanted) just before the Midnight Christmas Mass. It is with true love and moving faith to be able to remember this tradition where it still exists.

The Proclamation of the Birth of Christ, Kalenda Proclamation, or Christmas Proclamation, is a chant sung before the Midnight Mass for Christmas in the Roman Rite of the Catholic Church. The long text is a timeline, in which each verse represents the years from an historical event, either secular or religious, until birth of Jesus Christ, and the number of years – expressed in centuries or years – decreases until the day of the first Christmas.

Ceremonial

Originating from the Roman Martyrology, traditionally read during the hour of Prime, the proclamation places the birth of Christ "within the context of salvation history."1 Prime was suppressed as part of the liturgical reforms following Vatican II, but Pope John Paul II restored the usage of the Proclamation during the 1980 Papal Christmas Midnight Mass. Since then, many parishes re-instituted the Proclamation as well.

In the Ordinary Form of the Roman Rite, the Christmas Proclamation is chanted during Midnight Mass.

Octavo Kalendas Januarii Luna N.
Innumeris transactis saeculis a creatione mundi, quando in principio Deus creavit caelum et terram et hominem formavit ad imaginem suam; permultis etiam saeculis, ex quo post diluvium Altissimus in nubibus arcum posuerat, signum fœderis et pacis; a migratione Abrahæ, patris nostri in fide, de Ur Chaldæorum saeculo vigesimo primo; ab egressu populi Israël de Ægypto, Moyse duce, saeculo decimo tertio; ab unctione David in regem, anno circiter millesimo; hebdomada sexagesima quinta, juxta Danielis prophetiam; Olympiade centesima nonagesima quarta; ab Urbe condita anno septingentesimo quinquagesimo secundo; anno imperii Cæsaris Octaviani Augusti quadragesimo secundo; toto orbe in pace composito, Jesus Christus, æternus Deus æternique Patris Filius, mundum volens adventu suo piissimo consecrare, de Spiritu Sancto conceptus, novemque post conceptionem decursis mensibus, in Bethlehem Judæ nascitur ex Maria Virgine factus h*m*: Nativitas Domini nostri Jesu Christi secundum carnem. - Proclamation of the Birth of Christ

The Nativity of Our Lord Jesus Christ according to the flesh.

"Christmas" sounds so much better instead of "Nativitatimas" and it also keeps Christ in Christmas, just like Michaelmas keeps St. Michael in St. Michael's Day and Martinmas keeps St. Martin in St. Martin Day.