In other words, what were the zeitgeist and practice of the holiday before Victorian culture codified what we know today?
I have a couple of earlier answers that might interest you! I'll copy over the one I think is more relevant, on Christmas in Europe during the late Middle Ages, and link the others below. :)
Felicem diem nativitatis!
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Late medieval Christmas in the West was a month-ish long overlap of two seasons that unavoidably ended up blending together: (A) the not-entirely-religious December festivities, which technically ran from St. Nicholas Day through the Feast of the Innocents (Dec. 6-Dec. 28), but were in practice extended due to (B) the liturgical celebration of Christmas, which spanned Christmas to the Epiphany. As with today, there was a mix of sacred solemnity and ribald revelry.
Most basically, the Advent and Christmas seasons were characterized during Mass by their liturgies (well, like every other day). That is to say, specific Bible readings and hymns would be used. While the service itself was in Latin, which the (vast) majority of people would only have learned a little of via osmosis over the years, by the 14th-15th century the sermon would have been in the vernacular, including an explanation of the Gospel reading.
The 15th century also saw a move towards elevating the importance of Christmas Mass. At the Fourth Lateran Council in 1215, the Church had decreed that all Christians of both sexes had to receive the Eucharist once a year, at Easter, and make their confession before that (because receiving the sacrament in a state of sin is a mortal sin). Over the 15th century, however, we see a move towards local dioceses and synods mandating Eucharistic reception multiple times per year. Most frequently, this meant simply adding Christmas as a date when the sacrament was offered to the laity.
The theological-devotional innovation of the later Middle Ages is a focus on the humanity of Christ, of his life on Earth. Most famously this meant a concentration on the Passion, but the cycle of events surrounding the birth of Jesus (including his apocryphal circumcision) also garnered its devotional practices. Francis of Assisi in the 13th century did not actually invent the living creche or Nativity display, but his and his Franciscan Order certainly popularized it.
Devotion to the Christ-child, which is to say the infant Jesus, also led to one of the most medieval religious devotions out there: cradle-rocking. Medieval people, especially nuns in monasteries, performed elaborate rituals of rocking, dressing, and mock-nursing dolls that represented Jesus. Here is a famous and insanely ornate crib from 15th century Germany. You can see how it is carved and painted to resemble a cathedral, making the theological point that 'today' Christ is nurtured and protected by the Church. While cradle-rocking and Christ-child devotion was not limited to the Christmas season, especially by C15, the writings of holy women visionaries like Gertrude of Helfta (d. 1302) suggest it carried more significance and holiness in the Christmas season.
Thus far I have discussed a sacred and solemn season by 1500. During the English Interregnum 150 years later, of course, the English government will clamp down on the wildly out-of-control Christmas festivities (and not, we know, entirely effectively), following in the footsteps of the Spanish Catholics and Swiss Calvinists in C16. While, true, the Calvinists certainly weren't tucking Christ-child dolls into cathedral cribs, it wasn't sermon-going and Church attendance that these disparate early modern governments denounced. Instead, it was the libertates Decembris or Liberties of December: a month-long celebration with roots in pagan antiquity, that by the late Middle Ages had wedded itself obviously to the Christmas season and taken on new life.
12th century Parisian theologian Jean Beleth laid out the tradition and its origins:
Now the license which is then permitted is called Decembrian, because it was customary of antiquity among the pagans that during this month slaves and serving-maids should have a sort of liberty given them, and should be put upon an equality with their masters, in celebrating a common festivity.
Yes, this is the famous/infamous feast of fools as part of the verkehrtes Welt or world upside down tradition: a time when lay people and lower clergy could safely and obscenely parody and ridicule priests, bishops, and the Church. In legal codes in Spain, at least, the December liberties kicked off with the Feast of St. Nicholas--the patron of students and children. This is probably typical of elsewhere in Europe, too, because the 'lord of misrule' elected to preside over festivities in many areas was known as a "Bachalarius"--originally, this meant apprentice, but by the late Middle Ages it had come to mean university student (a la bachelor's/baccalaureate degree today). In less intellectually-centric parishes, it is the more well known 'boy bishop' tradition.
There are some great anecdotes of this festum stultorum, feast of fools. In Paris, lower clergy (deacons, clerks, Mass-servers) were known to burn shoes instead of incense in the censers used during Mass. People would play drinking games inside churches and roll dice on the altar. Public processions would parody the solemn penitential and Eucharistic processions of the rest of the year, like singing satirical (and dirty, apparently) hymns and ending in massive feasting. And apparently it was a major breach of medieval Christianity to--gasp--hold the liturgical book, that you had memorized long ago, upside down.
And, where there are Christmas parties, there must also be Christmas party-poopers. Already in the 13th century, popes Innocent III and Gregory IX sought to ban the December/Christmas festivities. A common complaint from civic and religious authorities alike is, interestingly, that lay people and lower clergy are mocking priestly garments.
So already in the Middle Ages, Christmas was a blend of religious solemnity and not-really-religious festivity, singing specific songs, eating too much, acknowledging pagan roots, and people complaining about the pagan roots taking over.
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