Learn Hardcore Russian: Моя семья собирается отметить праздник дома. - My family is going to celebrate the holiday at home.

семья is grammatically singular, feminine (like книга).
Therefore, the verb must also be 3rd person singular feminine:
- Моя семья собирается… – My family is going…

Saying Моя семья собираются… is usually considered incorrect in standard Russian (though you may hear it colloquially from some speakers).

What exactly does собирается mean here? Is it like “is going to,” “plans to,” or “is preparing to”?

In this context, собираться + infinitive means “to be going to / to intend to / to plan to do something.”

Literal meaning of собираться:

собираться (reflexive) = to gather, to assemble; also: to get ready, to prepare.
With an infinitive, it expresses intention or near future:
- Я собираюсь уехать. – I’m going to leave / I intend to leave.
- Моя семья собирается отметить праздник. – My family is going to celebrate the holiday.

So it’s very close to English “to be going to do something”, with a nuance of intent or plan rather than just prediction.

Why is отметить used instead of отмечать? What’s the difference?

This is the aspect difference:

отмечать – imperfective: to celebrate (habitually, in progress, general process)
отметить – perfective: to celebrate (once, as a single complete event)

In the sentence:

Моя семья собирается отметить праздник дома.
The family is going to have one specific celebration (this upcoming holiday).
→ So the perfective отметить is appropriate.

Compare:

Моя семья всегда отмечает праздники дома.
My family always celebrates holidays at home. (habit, repeated → imperfective)

Why is отметить in the infinitive form here?

After собираться (when it means “to be going to / intend to”), Russian requires the infinitive of the action verb:

собираться + infinitive
- собирается уехать
- собирается поужинать
- собирается отметить праздник

So отметить is the infinitive “to celebrate (once)”, just like English “to celebrate” after “is going to.”

What case is праздник in, and why doesn’t its form change?

Here, праздник is in the accusative case, as the direct object of отметить:

Who is going to do something? – (Моя) семья
What are they going to do? – отметить что? – праздник (accusative)

For masculine inanimate nouns, the accusative singular equals the nominative singular:

nominative: праздник
accusative: праздник (same form)

So it has changed case (syntactically), but the form is identical.

What does праздник mean exactly? Is it “holiday,” “party,” or “celebration”?

праздник is a general word meaning:

a holiday (official or traditional):
- Новый год – мой любимый праздник.
a festive occasion / celebration:
- У нас сегодня праздник. – We have a celebration today.

It does not automatically mean “party” in the sense of a social gathering with friends; for that, Russian more often uses вечеринка.

In the sentence, отметить праздник дома is naturally understood as:

“celebrate the holiday at home”
(and which holiday is usually clear from context).

Why is it дома and not в доме or домой?

These are different forms with different meanings:

дома – at home (location, no movement):
- Мы будем дома. – We will be at home.
- отметить праздник дома – to celebrate the holiday at home.
в доме – in the house (in a building); more physical/locational:
- Could be any house, not necessarily “one’s home.”
- В доме холодно. – It is cold in the house.
домой – (to) home (direction, movement):
- Мы пойдём домой. – We will go home.

Here, there is no movement; it’s a location of the event. So дома (“at home”) is correct.

Can the word order be changed? For example, can I say Моя семья собирается дома отметить праздник?

Yes, Russian word order is quite flexible. All of these are grammatically possible:

Моя семья собирается отметить праздник дома.
(neutral; standard order)
Моя семья собирается дома отметить праздник.
Slight extra emphasis on at home (where, not what).
Моя семья дома собирается отметить праздник.
Emphasizes that the family is already at home when they are getting ready to celebrate.

The basic meaning remains the same, but moving дома can shift the focus or nuance of what is being highlighted.

How would I say “My family celebrates holidays at home” (as a habit, usually), not just this one time?

For a habitual / repeated action, you need the imperfective aspect:

Моя семья отмечает праздники дома.

Notes:

отмечает – imperfective: celebrates (regularly / generally).
праздники – plural: holidays (in general, more than one).

Your original sentence with собирается отметить refers to one upcoming celebration, not a general habit.

How does собирается + infinitive compare to using будет for the future, like in Моя семья будет отмечать праздник дома?

They both refer to the future, but with different nuances:

Моя семья собирается отметить праздник дома.
- Focus on intention / plan.
- Very similar to “My family is going to celebrate the holiday at home.”
Моя семья будет отмечать праздник дома.
- Focus on the future action itself, more neutral or descriptive.
- Depending on context, can sound:
  - like a future arrangement: will be celebrating (this time), or
  - like a general future habit if used with adverbs (e.g. всегда, теперь).

So собирается emphasizes planning / intention, whereas будет + verb just expresses future tense.

What is the difference between собираться and собирать without -ся?

These are two different verbs:

собирать (non-reflexive):
- to gather/collect something:
  - собирать грибы – to pick mushrooms
  - собирать деньги – to collect money
- to assemble:
  - собирать мебель – to assemble furniture
собираться (reflexive, with -ся):
- to gather (people come together):
  - Мы собираемся у друзей. – We gather at our friends’ place.
- to get ready / prepare:
  - Я собираюсь на работу. – I’m getting ready for work.
- infinitive
  : to intend / to be going to:
  - Я собираюсь поехать. – I’m going to go.
  - Моя семья собирается отметить праздник. – My family is going to celebrate the holiday.

So in your sentence, собирается is reflexive and means “is going to / intends to”.