Why is the The Nativity called "Christmas"? Why was the the Feast of the Nativity of Jesus called "Christmas" and not some other feast day of Jesus?  There are lots of other feast days that have more or less fallen by the wayside etymologically speaking (Michaelmas, Candlemas, etc...), but Christmas has succeeded as the English name of the celebration. 

There are several other feast days for Jesus and one, namely the Feast of the Baptism of Jesus, would seem more appropriate as Christmas. Nativimas seems like it would have been a more appropriate name for the holiday.
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 homo: 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.