Learn Hardcore Russian: В праздник мы идём в парк с семьёй. - On the holiday we go to the park with the family.

в
- time expression in the accusative can mean “on / at (a time)”:
  - в понедельник – on Monday
  - в августе – in August
  - в праздник – on the holiday

So here в does not mean into or in a place, but on (that day).

Why is праздник in the singular, not plural like в праздники?

В праздник normally refers to one specific holiday:

В праздник мы идём в парк – On the (this/that) holiday we go to the park.

If you want to say you usually do this during holidays in general, you can say:

В праздники мы ходим в парк. – On holidays (in general) we (usually) go to the park.

So:

Singular в праздник – one particular holiday.
Plural в праздники – holidays in general / on holidays.

Why is it идём and not ходим or пойдём?

All three exist, but they mean different things:

идём – we are going (on foot) now / this time
- One-time, in-progress or planned event.
- В праздник мы идём в парк – On the holiday we are going to the park (this specific time).
ходим – we go (on foot) regularly, habitually
- В праздники мы ходим в парк – On holidays we (usually) go to the park.
пойдём – we will go (on foot), future tense
- В праздник мы пойдём в парк – On the holiday we will go to the park.

In your sentence the idea is: On the holiday, we’re going (this time), so идём works.

Why is the present tense идём used if this is about the future?

Russian often uses the present tense for planned future actions, especially when the time is clearly stated:

Завтра мы идём в кино. – Tomorrow we are going to the cinema.
В праздник мы идём в парк. – On the holiday we are going to the park.

The time phrase (в праздник, завтра) tells you it’s future, even though the verb form is present. This is similar to English We are going to the park tomorrow.

Why is it в парк and not на парк?

With most closed or defined spaces (buildings, parks, rooms, cities), Russian uses в for to / into:

в парк – to the park
в школу – to school
в библиотеку – to the library

На is used with many open surfaces, events, activities, institutions as “events”:

на стадион – to the stadium
на концерт – to a concert
на работу – to work

A park is treated as a space you go into, so it’s в парк.

What case is парк in, and why does it look like the nominative?

Парк here is in the accusative case, because it’s the destination after в (to where?):

идти куда? – в парк

But парк is a masculine, inanimate noun, and for this group nominative = accusative in form:

Nom.: парк – a park
Acc.: в парк – to the park

So it looks the same, but grammatically it’s accusative.

Why is it с семьёй and not с семья or something else? What case is that?

С семьёй uses the instrumental case after the preposition с when it means with (in company with):

с кем? – with whom?
с семьёй – with (the) family

Семья (family) declines:

Nominative: семья – family
Instrumental: семьёй – with (a/the) family

So семьёй is the instrumental singular of семья required by с = with.

What is that -ёй ending in семьёй? How is it pronounced?

The ending -ёй is a common instrumental singular ending for feminine nouns like семья:

семья → семьёй (syem-YÓY) – with (the) family
земля → землёй – with the earth / with land

Pronunciation:

Stress is on the last syllable: семь-ЙОЙ
The final letter й is a short y sound, similar to the y in boy.

Does с семьёй automatically mean “with my/our family”? Where is “my / our”?

In Russian, possessives (мой, наш, etc.) are often omitted when it’s obvious from context whose family it is.

Я иду в парк с семьёй. – I’m going to the park with my family.
Мы идём в парк с семьёй. – We’re going to the park with our family.

If you need to be explicit, you can say:

с моей семьёй – with my family
с нашей семьёй – with our family
со своей семьёй – with my/our own family (reflexive, emphasizes it’s your own family).

Could I drop мы and just say В праздник идём в парк с семьёй?

Yes. In Russian the subject pronoun is often dropped, because the verb ending already shows the person and number:

Идём в парк. – We’re going to the park.
В праздник идём в парк с семьёй. – On the holiday, we’re going to the park with (our) family.

Keeping мы adds a bit of clarity or emphasis, but both versions are grammatically correct and natural.

Can the word order be different, for example Мы в праздник идём в парк с семьёй? Does it change the meaning?

Yes, Russian allows relatively free word order. All of these are possible:

В праздник мы идём в парк с семьёй.
Мы в праздник идём в парк с семьёй.
Мы идём в парк с семьёй в праздник.

The basic meaning stays the same, but the focus can shift slightly:

Starting with В праздник highlights the time: On the holiday (what happens?)
Starting with Мы highlights the subject: We (not someone else) are going…

In neutral speech, В праздник мы идём… sounds very natural.

If we go by car or bus, should I still use идём, or something else?

Use идём when you mean going on foot.
If you’re going by transport, use ехать / ехать:

В праздник мы едем в парк с семьёй. – On the holiday we are going (by transport) to the park with (our) family.

So:

On foot: идём
By vehicle (car, bus, train, etc.): едем

How is the whole sentence stressed?

Stresses:

В пРа́здник – stress on ПРА́З (first syllable: ПРАЗ-дник)
мы – no stress issue (one syllable)
идём – ид-ЁМ
в парк – one syllable, stressed as is
с семьёй – с семь-ЙÓЙ

So: В Пра́здник мы идём в парк с семь-ЙÓЙ.