Learn Hardcore German: Zu Weihnachten essen wir zusammen im Wohnzimmer. - At Christmas we eat together in the living room.

zu Weihnachten – very common; sounds neutral and idiomatic.
an Weihnachten – also correct; slightly more like “on Christmas (day/at Christmastime)”.
Weihnachten (without a preposition) – common too, especially in spoken German:
- Weihnachten essen wir zusammen im Wohnzimmer.

All three are possible. The difference is subtle:

zu Weihnachten and an Weihnachten both focus on the general time of the holiday.
Bare Weihnachten feels a bit shorter and more colloquial.

What you normally do not say is am Weihnachten. You would use am only with Weihnachtstag (Christmas Day):

Am ersten Weihnachtstag essen wir …

Why is there no article before Weihnachten? Shouldn’t it be zu dem Weihnachten or something?

No article is used because Weihnachten here is treated as the name of a holiday, similar to Christmas in English.

In German, names of holidays usually appear without an article in time expressions:

zu Weihnachten – at Christmas
an Ostern – at Easter
an Silvester / zu Silvester – at New Year’s Eve

Saying zu dem Weihnachten would sound wrong and unnatural in standard German.
So the pattern you want is simply: zu + holiday name (no article).

What case is Weihnachten after zu? I thought zu takes the dative, but I can’t see it.

You’re right that zu is normally followed by the dative case.
Here, too, Weihnachten is in the dative, it’s just not visible:

There is no article to show the case.
The noun Weihnachten doesn’t change form between nominative/accusative/dative.

So grammatically it’s still:

zu
- dative → zu Weihnachten

This is similar to other time expressions where you don’t see the dative ending clearly, for example:

am Abend (actually an + dem Abend)
zu Ostern (dative, but no visible ending)

Why does the verb essen come in second position after Zu Weihnachten? In English we say “At Christmas we eat…”.

German has the verb-second rule (V2) in main clauses:

The finite verb (here: essen) must be in second position.
First position can be the subject, but it can also be any other element (time, place, object, etc.).

In the sentence:

Zu Weihnachten = first element
essen = finite verb, must come second
wir zusammen im Wohnzimmer = the rest, after the verb

So:

Zu Weihnachten essen wir zusammen im Wohnzimmer. ✅
(Time element first, verb second, subject after verb)

If you start with the subject instead, it still obeys the same rule:

Wir essen zu Weihnachten zusammen im Wohnzimmer. ✅
(Subject first, verb second, time later)

Both are correct; the difference is mostly about emphasis and style.

Can I also say Wir essen zu Weihnachten zusammen im Wohnzimmer? Is there a word order rule for the time and place parts?

Yes, Wir essen zu Weihnachten zusammen im Wohnzimmer is completely correct.

A common guideline for German word order is:

Time – Manner – Place (often abbreviated as TMP)

In your original sentence:

Zu Weihnachten = Time
zusammen = Manner (how? together)
im Wohnzimmer = Place

So:

Zu Weihnachten essen wir zusammen im Wohnzimmer.
→ Time (fronted) – Verb – Subject – Manner – Place

In the alternative:

Wir essen zu Weihnachten zusammen im Wohnzimmer.
→ Subject – Verb – Time – Manner – Place

Both follow the TMP idea within the part after the verb.
You could also front the place for emphasis:

Im Wohnzimmer essen wir zu Weihnachten zusammen.

That’s still correct, but now you are emphasizing im Wohnzimmer more strongly.

What exactly does zusammen mean here, and where can it go in the sentence?

zusammen means “together”.

It describes how you eat (the manner), so it’s part of the Manner element in the TMP rule.

Typical positions:

Zu Weihnachten essen wir zusammen im Wohnzimmer.
Zu Weihnachten essen wir im Wohnzimmer zusammen.
Wir essen zu Weihnachten zusammen im Wohnzimmer.

All of these are fine. Native speakers most often put zusammen relatively close to the verb or the subject, but it is flexible.

You would not normally put it at the very beginning alone:

Zusammen essen wir zu Weihnachten im Wohnzimmer. – possible, but sounds like you’re strongly emphasizing together (e.g. “It’s together that we eat at Christmas…”).

What does im Wohnzimmer literally mean, and why is it dative?

im is a contraction of in + dem. So:

im Wohnzimmer = in dem Wohnzimmer = “in the living room”

The preposition in can take dative or accusative, depending on meaning:

Dative = location, “where?”
- Wir essen im Wohnzimmer. – We eat in the living room (we are there).
Accusative = movement, “where to?”
- Wir gehen ins Wohnzimmer. – We go into the living room.
  (ins = in + das, accusative)

In your sentence, the meaning is location (we are in the living room while we eat), so dative is used: im Wohnzimmer.

Could I say im Esszimmer instead of im Wohnzimmer? Does the grammar change?

Yes, you can just swap the noun; the grammar stays the same:

Zu Weihnachten essen wir zusammen im Esszimmer.
(Esszimmer = dining room)

It’s still:

im = in + dem (dative)
Esszimmer = neuter noun → dem Esszimmer → im Esszimmer

Is essen here present tense only for right now, or can it mean something like “we will eat” at Christmas?

German Präsens (present tense) is quite flexible. It can express:

Habitual actions
- Zu Weihnachten essen wir zusammen im Wohnzimmer.
  → Every Christmas, we eat together in the living room.
Planned future actions (especially with a time expression)
- Zu Weihnachten essen wir zusammen im Wohnzimmer.
  → This coming Christmas, we’ll eat together in the living room.

Because Zu Weihnachten is a clear time expression, listeners can understand from context whether you mean every year or this time in the future. You don’t have to use a special future tense.

Why is it wir and not uns? I thought uns meant “we/us”.

German distinguishes between subject and object forms, just like English we vs. us:

wir = subject form (“we”) – nominative
uns = object form (“us”) – accusative/dative

In the sentence:

wir is the subject (who is doing the eating) → needs nominative:
- Wir essen … – We eat …

You would only use uns if it’s an object, for example:

Sie lädt uns zu Weihnachten ein. – She invites us for Christmas.
Zu Weihnachten kochen sie für uns. – At Christmas they cook for us.

So wir is correct here because it is the “doer” of the action.

Are there any commas missing in Zu Weihnachten essen wir zusammen im Wohnzimmer?

No commas are needed here.

It’s a simple main clause with:

a fronted time expression (Zu Weihnachten)
the verb (essen) in second position
the subject (wir) and some adverbials (zusammen im Wohnzimmer)

German does not put a comma between a fronted element and the rest of the clause, so:

Zu Weihnachten essen wir zusammen im Wohnzimmer. ✅
Zu Weihnachten, essen wir zusammen im Wohnzimmer. ❌ (comma is wrong)