Minha irmã vai tirar férias em janeiro.

Because minha has to agree with irmã, which is a feminine singular noun.

In Portuguese, possessives agree with the thing/person possessed, not with the speaker.

• meu irmão = my brother
• minha irmã = my sister
• meus irmãos = my brothers / my siblings
• minhas irmãs = my sisters

So minha matches irmã.

Both minha irmã and a minha irmã are possible.

In Brazilian Portuguese, the definite article before a possessive is common but not required in many contexts.

So these both work:

• Minha irmã vai tirar férias em janeiro.
• A minha irmã vai tirar férias em janeiro.

Without the article, the sentence can feel a little lighter or more neutral. With the article, it can sound a bit more conversational in many regions of Brazil. The exact preference varies by speaker and region.

Vai tirar is the near future form:

• vai = goes / is going
• tirar = to take

So vai tirar férias literally looks like is going to take vacation, and naturally means is going to take time off / is going on vacation.

This structure is very common in Portuguese:

• vou estudar = I’m going to study
• vamos sair = we’re going to go out
• ela vai viajar = she’s going to travel

Yes, tirar often means to remove, to take out, or to take, but in Portuguese it is also used in some set expressions.

Tirar férias is an idiomatic expression meaning:

• to take vacation
• to take time off
• to go on leave

So even if the literal logic feels odd from English, it is the normal expression.

Other useful vacation expressions are:

• estar de férias = to be on vacation
• entrar de férias = to start vacation
• voltar de férias = to come back from vacation

In this meaning, férias is normally used as a plural noun in Portuguese, even when English might use singular vacation.

So you say:

• as férias
• minhas férias
• tirar férias

Not normally:

• a féria ❌
• minha féria ❌

Also, férias is feminine plural, which is why you would say:

• boas férias = have a good vacation
• férias longas = long vacation

With months, Portuguese normally uses em:

• em janeiro = in January
• em fevereiro = in February
• em julho = in July

You do not usually use an article with month names in this kind of expression, so no janeiro is not the normal form here.

So:

• em janeiro ✅
• no janeiro ❌

No. In Portuguese, names of months are normally written with lowercase letters:

• janeiro
• fevereiro
• março

They are only capitalized if they begin a sentence or are part of a title that follows capitalization rules.

So em janeiro is correct, not em Janeiro.

Irmã is pronounced approximately eer-MAHN, but with a nasal ending rather than a clear n sound.

A rough IPA version is /iʁˈmɐ̃/.

A few pronunciation notes:

• The i sounds like ee in see
• The r in Brazilian Portuguese is often pronounced from the throat, especially at the beginning of a syllable
• The ã is nasal: you let air pass through the nose while saying the vowel

The tilde ~ over ã marks that nasal sound.

Yes. That is grammatically correct.

• vai tirar = common spoken future
• tirará = simple future, often more formal or written

So:

• Minha irmã vai tirar férias em janeiro. = very natural in everyday Brazilian Portuguese
• Minha irmã tirará férias em janeiro. = correct, but more formal or less conversational

In normal speech, Brazilians usually prefer vai + infinitive.

They are not the same.

• férias = vacation, time off from work or school, usually a longer period
• feriado = a holiday, especially a public holiday

Examples:

• Minha irmã vai tirar férias em janeiro. = My sister is going to take vacation in January.
• O feriado é na sexta-feira. = The holiday is on Friday.

So if someone is away from work for a week or two, that is férias, not feriado.

Yes. Tirar férias can cover both ideas.

Depending on context, it may mean:

• she will be on vacation
• she will take leave from work
• she will take time off
• she will go away on vacation

The exact nuance comes from the situation, not from the grammar alone. If needed, Portuguese can be more specific:

• Minha irmã vai estar de férias em janeiro. = My sister will be on vacation in January.
• Minha irmã vai viajar em janeiro. = My sister is going to travel in January.