Learn Hardcore Portuguese (Portugal): Ao fim de semana, fazemos voluntariado num abrigo para animais. - At the weekend, we do volunteering at an animal shelter.

Ao fim de semana (literally “at weekend”) is the most typical way in European Portuguese to say “at weekends / on weekends” in the sense of a habitual action.
- Ao = a + o (preposition a “to/at” + definite article o “the”).
No fim de semana (literally “on the weekend”) is more often used for a specific weekend:
- No fim de semana, vou visitar os meus pais.
  “This weekend / on the weekend, I’m going to visit my parents.”

So in your sentence, because it describes a regular activity, “Ao fim de semana, fazemos voluntariado…” is the most natural choice in Portugal.

Why is it “fim de semana” in the singular when English uses “weekends” in the plural?

Portuguese often uses the singular with “ao” to express a general, repeated time:

ao fim de semana = at weekends
ao domingo = on Sundays
ao jantar = at dinner(time)

You can use the plural:

aos fins de semana = on weekends

…but “ao fim de semana” (singular) is very common in European Portuguese and still means “on weekends” in general, not just one weekend.

What exactly does “ao” mean here?

“Ao” is a contraction:

a + o → ao
- a = “to, at”
- o = masculine singular definite article “the”

So “ao fim de semana” is literally “to/at the weekend”, but in idiomatic English we say “at weekends / on weekends.”

What does “num” mean, and how is it formed?

“Num” is another contraction:

em + um → num
- em = “in, on, at”
- um = masculine singular indefinite article “a / one”

So:

num abrigo = “in a shelter”

If you wanted to say “in the shelter”, you would use the definite article:

em + o → no
no abrigo = “in the shelter”

Why is it “num abrigo” (“in a shelter”) and not “no abrigo” (“in the shelter”)?

The choice between “num” and “no” is about definiteness:

num abrigo = in a shelter (unspecified shelter, one of many)
no abrigo = in the shelter (a specific, known shelter)

In English, “we volunteer at an animal shelter” usually doesn’t refer to a clearly identified, previously mentioned place, so “num abrigo para animais” with the indefinite article sounds more natural in Portuguese.

Why do we say “fazemos voluntariado” and not just a verb equivalent to “we volunteer”?

In English, “to volunteer” is a verb. In Portuguese (especially European):

The more natural pattern is verb + noun:
- fazer voluntariado = literally “to do volunteer work”
  → idiomatic English: “to volunteer”

There is a verb “voluntariar(-se)”, but:

It’s less common in everyday European Portuguese.
“fazer voluntariado” or “ser voluntário” are more idiomatic.

So:

Ao fim de semana, fazemos voluntariado…
= “At weekends, we volunteer…”

You could also say:

Ao fim de semana, somos voluntários num abrigo para animais.
(“At weekends, we are volunteers at an animal shelter.”)

Could I say “somos voluntários num abrigo para animais” instead? What’s the difference from “fazemos voluntariado”?

Yes, you can, and it’s correct.

fazemos voluntariado
Focuses on the activity: “we do volunteer work”.
somos voluntários
Focuses on our role/status: “we are volunteers”.

In most contexts, they’re interchangeable:

Ao fim de semana, fazemos voluntariado num abrigo para animais.
Ao fim de semana, somos voluntários num abrigo para animais.

Both are natural and mean essentially the same thing.

Why is there no subject pronoun “nós” before “fazemos”?

Portuguese is a “pro-drop” language: you can (and usually do) omit subject pronouns when the verb ending already makes the subject clear.

(Nós) fazemos voluntariado…
- fazemos is the 1st person plural form, so it already tells you “we”.

You use the pronoun nós mainly when you want to:

Emphasize contrast:
Nós fazemos voluntariado, eles não.
“We volunteer, they don’t.”
Avoid ambiguity (less common with “nós” because its ending is quite clear).

In neutral sentences like yours, leaving out “nós” is more natural.

Could I move “ao fim de semana” to the end of the sentence? Is the comma necessary?

Yes, the time expression can move:

Ao fim de semana, fazemos voluntariado num abrigo para animais.
Fazemos voluntariado num abrigo para animais ao fim de semana.

Both are correct and natural.

About the comma:

When the time expression comes at the beginning, a comma is standard:
- Ao fim de semana, fazemos voluntariado…
When it’s at the end, usually no comma:
- Fazemos voluntariado num abrigo para animais ao fim de semana.

What’s the difference between “abrigo para animais” and “abrigo de animais”?

Both are understandable, but there’s a nuance:

abrigo para animais
Literally “shelter for animals”
→ Emphasizes purpose: the shelter exists for animals.
→ This is the most idiomatic phrase for “animal shelter”.
abrigo de animais
Literally “shelter of animals”
→ More ambiguous: could mean a place belonging to animals, or a shelter where there happen to be animals.

For the institutional idea of an animal shelter, use “abrigo para animais”.

Why is the preposition “para” used here instead of “a” or “de”?

In “abrigo para animais”:

para expresses purpose / intended recipient:
- um abrigo para animais = “a shelter for animals” (designed for them).

Other prepositions would change the meaning:

abrigo de animais: “shelter of animals” (more about possession/association, less about purpose).
abrigo a animais: not natural Portuguese in this context.

So “para” is the right choice to show that the shelter’s function is to take in animals.

How would this sentence typically be said in Brazilian Portuguese?

A natural Brazilian Portuguese version would be something like:

Nos fins de semana, fazemos trabalho voluntário em um abrigo de animais.

Main differences:

Nos fins de semana instead of Ao fim de semana
(Brazil prefers “fim de semana” in the plural fins for habitual actions.)
fazemos trabalho voluntário instead of fazemos voluntariado
(“trabalho voluntário” is more common in Brazil, though voluntariado is also understood.)
em um abrigo de animais instead of num abrigo para animais
(Brazil more often separates em um; “abrigo de animais” is frequent there.)

Your original sentence is clearly European Portuguese in style.