Learn Hardcore Russian: Вся семья собралась дома, чтобы отметить его юбилей. - The whole family gathered at home to celebrate his anniversary.

семья is a feminine singular noun (it ends in -я and is grammatically feminine).
Therefore, in the past tense the verb must be feminine singular: собралась (she gathered).

Even though in meaning it’s many people, grammatically it’s one unit.

Compare:

Вся семья собралась. – The whole family gathered. (singular)
Все члены семьи собрались. – All the family members gathered. (plural собрались)

Why is the verb reflexive собралась and not something like собрала?

Russian has two different verbs:

собрать (non‑reflexive, transitive) – to gather/collect something or someone.
- Он собрал книги. – He collected the books.
собраться (reflexive, intransitive) – to gather, to come together.
- Семья собралась. – The family got together.

In your sentence, the family is not collecting something; they are themselves coming together. That is why the reflexive verb собраться is used: семья собралась.

Why do we say вся семья, not все семья or все семьёй?

It is about agreement and case:

семья is feminine singular nominative.
The word вся is the feminine singular form of весь (all, the whole), so it matches семья in gender and number: вся семья (the whole family).

The form все is plural and would need a plural noun:

все люди – all the people
все дети – all the children

семьёй is the instrumental case:

Мы поехали в отпуск семьёй. – We went on vacation as a family / with the whole family.

So:

вся семья – correct here (subject in nominative).
все семья – ungrammatical.
семьёй – a different case and meaning (as a family), not suitable as the subject here.

What is the difference between дома, домой and в доме?

These three forms are very common and easy to mix up:

дома – at home (location, where?)
- Я сейчас дома. – I am at home.
  In your sentence: собралась дома = gathered at home.
домой – (to) home (direction, where to?)
- Я иду домой. – I am going home.
в доме – in the house (inside a house, more literal/physical)
- Они сидят в доме. – They are sitting in the house.

In this sentence, the idea is that they all got together at home (their home base, family home), so дома is the most natural choice.

Why is there a comma before чтобы?

чтобы introduces a subordinate clause of purpose (a purpose clause).

In Russian, subordinate clauses are almost always separated from the main clause by a comma. Here:

Main clause: Вся семья собралась дома (The whole family gathered at home)
Subordinate purpose clause: чтобы отметить его юбилей (in order to celebrate his jubilee)

Because чтобы starts that subordinate clause, a comma before it is required:
..., чтобы отметить его юбилей.

How does чтобы with an infinitive work here? Could I say ... чтобы семья отметила его юбилей instead?

Here чтобы + infinitive (отметить) expresses purpose:

Вся семья собралась дома, чтобы отметить его юбилей.
The whole family gathered at home in order to celebrate his jubilee.

When the subject of the action in the subordinate clause is the same as in the main clause (the family gathered, and the family will celebrate), Russian often uses чтобы + infinitive.

You can also say:

Вся семья собралась дома, чтобы отметить его юбилей.
Вся семья собралась дома, чтобы семья отметила его юбилей.

The second version is grammatically possible but sounds heavier and less natural here, because the subject семья is repeated unnecessarily. Native speakers strongly prefer the version with just the infinitive.

Why is отметить (perfective) used, not отмечать?

This is about aspect:

отмечать – imperfective: to be celebrating, to celebrate regularly, to be in the process.
отметить – perfective: to celebrate (as one complete event), to mark an occasion.

In your sentence, the family gathered in order to perform a single, complete act of celebration (one event, not a habitual action). So the perfective отметить is used.

If you used отмечать here, it would sound odd or suggest a habitual action, which does not fit the context:

Семья каждый год собирается, чтобы отмечать его день рождения. – The family gathers every year to celebrate his birthday. (habitual → imperfective makes sense)

What exactly does юбилей mean? Is it just any birthday?

Юбилей is not just any birthday.

Typically it means:

a special anniversary, usually a round number:
- 25, 30, 40, 50, 60, 70 years, etc.
It can be:
- a person’s round‑number birthday,
- or the anniversary of an event, company, institution, marriage, and so on.

For an ordinary birthday, Russians usually say день рождения.
So this sentence suggests it is a special, significant birthday or anniversary, not just another regular year.

What does его mean here, and why that form instead of something like ему?

In this sentence, его is a possessive pronoun: его юбилей = his jubilee.

Important points:

его as a possessive (his) is invariable: it does not change for case, gender, or number:
- его юбилей – his jubilee
- его семья – his family
- его книги – his books
  The form is always его.
ему is a different word: the dative form of the personal pronoun (to him, for him):
- Ему 50 лет. – He is 50 years old. (literally: To him are 50 years)
- Я дал ему подарок. – I gave him a present.

So:

его юбилей – his jubilee (possessive, whose?)
ему – to him (dative, to whom?)

Can we change the word order, for example Семья вся собралась дома or Вся семья дома собралась?

Yes, Russian word order is relatively flexible, though not all versions sound equally natural.

Some possible variants:

Вся семья собралась дома, чтобы отметить его юбилей.
Neutral, standard, very natural.
Собралась дома вся семья, чтобы отметить его юбилей.
Puts a bit more emphasis on вся семья (it was the whole family that ended up gathered).
Вся семья дома собралась, чтобы отметить его юбилей.
Also possible; slight focus on the place дома.
Семья вся собралась дома, чтобы отметить его юбилей.
Acceptable in spoken Russian, with emphasis on вся (every member).

The original word order is the most neutral; moving words around usually adds a nuance of emphasis rather than changing the basic meaning.

What cases are used for the main nouns in this sentence?

Let’s break it down:

семья – nominative singular
It is the subject: who gathered? The family.
дома – formally can be seen as a special adverbial form meaning at home. Historically related to the genitive of дом, but in modern grammar it is usually treated as an adverb.
юбилей – accusative singular
It is the direct object of отметить: what to celebrate? The jubilee.
For inanimate masculine nouns, nominative and accusative singular look the same: юбилей / юбилей.
его – here functions as a possessive pronoun (his) modifying юбилей. Its form coincides with the genitive/accusative of the pronoun он, but in this position you can think of it simply as the possessive his.

Is there anything tricky about the pronunciation of семья and дома in this sentence?

Yes, two small points:

семья
- Stress on the second syllable: se‑мья → [сим‑ья].
- The м is soft because of the following я, and the я represents [йа].
дома
This word has two different stresses and meanings in Russian:
- дОма – at home (what we need here)
- домА – houses (plural nominative/accusative of дом)
In this sentence, the meaning is at home, so the stress is on the first syllable: дОма.