What is the origin of "Navidad" as the name of the season? How was it called before?

Question

Yesterday, at a birthday party, somebody had wrapped their gift with paper from an old, different Christmas gift (recycling FTW!).
The thing is that the paper still had the name of the former recipient written on it, so when the birthday boy received his gift and read an unknown name aloud with a puzzled look, there was a brief moment of confusion, until the gifter explained what had happened.

Then the following dialogue ensued:

TITA 1: "Tita 2" tiene un montón de papel de regalo antiguo guardado, ¡creo que alguno todavía pone "Navidad 1320" o algo así! ¡Jajaja!

TITA 2: No, del siglo catorce ya no me queda, lo gasté, voy ya por el siglo quince :D

(Todos ríen)

But I was left wondering: was Navidad called "Navidad" in the 14th century in Spain? Even today, most almanacs note the day as "Natividad del Señor", not "Navidad", so probably it wasn't.
So my question is:

How has Navidad been called through the centuries in Spanish?
And how did we end up with the current name?

Answer 1

The first Spanish dictionary, the Nebrija's from 1495, already registers the term Navidad as Natalis Christianus. So in the 15th century the term was already used. If you look up the term in the CORDE, we can also find texts from the 13th century:

Ende, vos rogamos e mandamos que vos plega desta avenençia e que la reçibades e que la otorguedes e que fagades cartas partidas por a. b. c., seelladas de vuestros siellos, entre vos e la abadessa e el convento sobredichos por que esta avenença sea firme e estable por yamas; e depues que las ovierdes fechas, enviadenolas a ocho dias depues desta Navidad primera que agora verna e enviadenos rogar e pidir que pongamos nuestro siello en ellas; e se lo assi fezierdes, gradirvolo hemos mucho e faredes y cosa que nos mucho plazera.

Anónimo, "Carta Real" [Documentos de Alfonso X dirigidos al Reino de León], 1259 (Spain).

---

E mandamos que los cavalleros que tovieren las maiores casas pobladas con mugieres e con fijos, e los que no tovieren mugieres con la compaña que oviere, desde ocho días antes de Navidad fasta ocho días después de Cinquesma, e tovieren cavallos e armas, e el cavallo de treinta maravedís arriva, e escudo e lanza e loriga e brafaneras e perpentte e capiello de ferro e espada, que no peche.

Anónimo, "Alfonso X concede franquicias y exenciones", 1256 (Spain).

So no changes from the 13th century until today. I have also found some texts using the variant Navidat:

Et los fijosdalgo que fueremos moradores en nuestro heredamiento destos logares nombrados que a vos damos que cortemos en los montes destos logares sobredichos en las tres Pasquas del anno, cada uno dos cargas de lenna cada Pasqua; et las Pasquas son estas: Navidat e Pasqua de Resurrection e Cinquaesma.

Anónimo, "Concordia" [Documentos de Alfonso X dirigidos a Castilla la Vieja], 1258 (Spain).

The word itself seems to have been in use even before, but as a common noun instead of as the name of the season:

Alli era Zacharias sacerdoth, e veno a él el angel e nuncio la navidat de so fijo Sant Juan baptista.

Almerich, "La fazienda de Ultra Mar", c1200 (Spain).

I cannot find references to Navidad or Natividad prior to that date, as texts referring to that event from the 12th century are written in Latin, so no clues about the origin of the term, other than what the DLE says:

Del lat. tardío nativĭtas, -ātis 'nacimiento'.

Also, Corominas suggests an intermediate form nadvidad happening in texts around 1200, but I have not found those texts. That would explain the origin from nativitas by softening the two t sounds.