I am not an expert but a native speaker. Weihnachten is kind of a strange word. Normally it is used without article. I think it might be already plural. (literally meaning holy nights in old german) But i have no idea. I am really trying to think of phrases with "Weihnachten" but I can only think of "Frohe Weihnachten" und "weiße Weihnachten" (white christmas) and "ich freue mich auf Weihnachten" (I am looking forward to christmas) in all of these Weihnachten is treated as plural. I think it is a word like people (Leute), which is always plural and there is no singular. But as I said, I am not an expert.
This is how Wiktionary explains it in its "Usage notes":
Weihnachtenis originally a plurale tantum with the singular meaning Christmas. This is still invariably used in wishes: Frohe, _gesegnete, _schöne, … Weihnachten! Otherwise the word is most often treated as a neuter singular: Weihnachten ist ein christliches Fest. (“Christmas is a Christian holiday.”) Particularly in Austria and Switzerland the plurale tantum may alternatively be used, then requiring the definite article: .Die Weihnachten sind ein christliches Fest. The neuter singular also has a true plural referring to Christmases in different years: Die letzten drei Weihnachten war er krank. (“He was sick for the past three Christmases.”)
The etimology notes there indicate the word comes…
From Middle High German wīhenahten (“Christmas”), from a dative plural ze den wīhen nahten (“in the holy nights”).
Check the links below for a little more complete info:
It's from Middle German Dativ pl. ze den wīhen nahten "in the Holy Nights." Modern equivalents would be weihen "to hallow" and Nachten "nights." There used to be a singular form that referred to Christmas specifically, but now they just use the plural for the days around Christmas and Christmas itself.
According to the website http://en.pons.com/, Merry Christmas is fröhliche Weihnachten!
The entry says: "fröhliche [o. geh gesegnete] Weihnachten! merry Christmas!" I had no idea what the "geh" meant. I looked it up, and found that "If it's from a dictionary, geh. usually stands for gehoben (elevated). If that is correct, it means that saying "gesegnete Weihnachten!" would be a very formal way of saying "Merry Christmas." I'm pasting the url for the dictionary entry below.
I've found the above descriptions of Weihnachten interesting regarding whether it is technically a plural form because the same is true of Yule, which in Anglo-Saxon 'géola' was a plural, meaning 'the festivities' from Proto-Germanic 'jehwlą', related to Latin 'iocus', a joke or play. The original Anglo-Saxon month names of December and January were 'Ǽrra Géola' and 'Æfterra Géola', the earlier and later Yule, and seems that this time described two months of winter rest from farm labour, it described periods (plural) of time matches with the idea of Weihnachten (Wighnights, or holy nights) being plural.
My first thought was, "Shouldn't it be Weinachten SIND im Dezember because I've only every seen Weihnachten with plural adjectives. But I see it can be treated as singular, and therefore Weihnachten ist im Dezember makes sense. If you mean Christmas Day. Maybe if you mean the whole Christmas season you could say Weihnachten sind im Dezember und (im) Januar. What do native German speakers think?