Na terça-feira à noite, eu vou ao teatro com minha prima.

Because na is the contraction of em + a.

• em can mean on / in / at, depending on context.
• a terça-feira = the Tuesday

So na terça-feira literally comes from em a terça-feira, and in natural Portuguese that becomes na terça-feira.

In this sentence, it means on Tuesday or on Tuesday evening. It usually refers to a specific Tuesday, not a repeated habit.

A very common way is:

• às terças-feiras à noite
• or toda terça-feira à noite

These mean every Tuesday night.

Compare:

• na terça-feira à noite = on Tuesday night (one specific Tuesday)
• às terças-feiras à noite = on Tuesday nights / every Tuesday night

Notice the plural in terças-feiras for the repeated meaning.

Because the names of weekdays from Monday to Friday in Portuguese are traditionally written with -feira attached by a hyphen:

• segunda-feira
• terça-feira
• quarta-feira
• quinta-feira
• sexta-feira

This is just the standard spelling. You need to learn them as fixed forms.

In Portuguese, days of the week are normally not capitalized unless they begin a sentence.

So:

• na terça-feira = correct
• na Terça-Feira = not standard

This is different from English, where Tuesday is always capitalized.

The à here shows crase, which is the contraction of two a sounds:

• a preposition a
• plus the feminine article a

So:

• a + a = à

In this expression, à noite is a fixed and very common way to say at night / in the evening, depending on context.

Important: this is a grave accent, not the same as the accent in words like avó.

Because Portuguese time expressions are not fully symmetrical. You just have to learn the common patterns:

• de manhã = in the morning
• à tarde = in the afternoon
• à noite = at night / in the evening

So even though English learners often expect one single pattern, Portuguese uses different prepositions here. These are best learned as set expressions.

Yes, you could say:

• Na terça-feira à noite, vou ao teatro com minha prima.

Portuguese often allows the subject pronoun to be omitted because the verb ending already shows the person:

• vou = I go / I’m going

That said, in Brazilian Portuguese, subject pronouns like eu are used quite often, so eu vou sounds very natural too. Including eu can add clarity or a slight sense of emphasis, but it is not required.

Because Portuguese, like English, often uses the present tense for a planned or scheduled future when the time is already clear.

Here, the time expression Na terça-feira à noite makes the future meaning obvious. So:

• eu vou ao teatro can mean I go to the theater or I’m going to the theater
• with Na terça-feira à noite, it clearly means a future plan

This is very common in everyday Portuguese.

Because ao is the contraction of:

• a = preposition
• o = masculine singular article

So:

• a + o = ao

The verb ir often takes the preposition a in standard Portuguese when talking about destination:

• ir ao teatro
• ir ao cinema
• ir à escola

So vou ao teatro means I’m going to the theater.

Yes, but there are differences.

vou ao teatro

This is the most standard form in careful Portuguese for going to the theater.

vou para o teatro

Also correct. It focuses a bit more on direction or destination: I’m heading to the theater.

vou no teatro

This is heard in Brazil in informal speech, but it is usually not considered the best choice in standard grammar for destination after ir.

So for learners, vou ao teatro is the safest and most standard option.

Portuguese uses definite articles with many place nouns more often than English learners expect.

That is why expressions like these are very common:

• ao teatro
• ao cinema
• ao mercado
• à praia

So in Portuguese, the article is part of the normal structure here. You should usually learn the whole chunk together:

• ir ao teatro not just
• ir teatro

In Brazilian Portuguese, both are possible:

• com minha prima
• com a minha prima

In Brazil, leaving out the article before a possessive is very common and sounds completely natural. Adding the article can sound a little more explicit, a little more formal, or just like a regional/style preference.

So in this sentence, com minha prima is perfectly normal Brazilian Portuguese.

Yes.

• prima = female cousin
• primo = male cousin

English uses cousin for both, but Portuguese usually marks gender. Since the sentence says minha prima, we know the cousin is female.

Yes, the word order is flexible.

For example, you could also say:

• Eu vou ao teatro com minha prima na terça-feira à noite.
• Na terça-feira à noite eu vou ao teatro com minha prima.

All of these are natural.

As for the comma: it is common to put a comma after a longer introductory time expression, as in:

• Na terça-feira à noite, eu vou ao teatro com minha prima.

It helps readability. In a shorter or more informal sentence, people may leave it out:

• Na terça-feira à noite eu vou ao teatro com minha prima.

So the comma is natural here, but not absolutely required in every context.