If you are interested in what actually happened during Christmas celebrations in Byzantium, we have accounts from the tenth-century Book of Ceremonies (commissioned by Constantine VII Porphyrogenitus) and a similar work by the fourteenth-century Pseudo-Kodinos. Both works are concerned with Byzantine court rituals and both talked about Christmas, as it was a major holiday throughout the history of the empire/Christianity. This only gives you the perspective of the upper class though, as I don't think we have any records of similar rituals from the lower classes. Unfortunately I can't access either texts at the moment, so I can't tell you any more about them - if you have access to a library, you can take a look at the recent translations of the former by A. Moffatt and M. Tall, and of R. Macrides for the latter. Hopefully a Middle Byzantium person will be around to give you a fuller answer soon!

I cannot say much about Macedonian era practices, but I assure you that Christmas was and is an important holiday in the Byzantine/Greek church. Since the twelfth century (and thus postdating your Macedonian timeframe, unfortunately) the Nativity feast has been preceded by a forty-day "Lesser Lent" -- not one of expiation, but of preparation to celebrate the incarnation.

How did the Byzantines celebrate Christmas? Was it a major holiday, or did others such as Easter have far greater significance? Were there any communal of familial celebrations? If so, do we have any idea what they might have looked like?

It was a major feast with a great procession and ceremony. As /u/shlin28 notes, the Book of Ceremonies (10th century) goes on at length about the preparation and stops made by an imperial procession which visited the two main churches in Constantinople, the Hagia Sophia and Church of the Holy Apostles (where the present day Fatih Mosque stands now). It was a big, flashy, public ritual where everyone wore their best clothing and had a role to play in the ceremony of it all. The Twelve Days of Christmas were each marked by a meal hosted by the Emperor with a rotating guest list of court, religious, civil, and military officials, some token representatives of the poor, the factions of the city, and even Muslim guests/prisoners.

I once read way more monastery constitutions than I ever wanted, and from my memory of those the monks/nuns would sometimes get a little extra food on Christmas (and other feast days) or maybe some spending money. That's all I know about non-nobles, but it does show that commemorating Christmas wasn't necessarily an upper class affair.