Learn Hardcore Portuguese (Portugal): As bancadas do estádio enchem-se de adeptos ao domingo. - The stands of the stadium fill up with fans on Sundays.

Bancada (plural bancadas) in this context means the stands or tiers of seating in a stadium – the big sections where people sit or stand to watch the game.

It is not the individual seats; those would be lugares or assentos.
So As bancadas do estádio ≈ The stands of the stadium.

Portuguese tends to use the plural bancadas because a stadium normally has several stands/sections.

Why does the sentence start with As bancadas and not just Bancadas without the article?

European Portuguese uses definite articles much more than English, even for general statements.

As bancadas do estádio enchem-se…
literally The stands of the stadium fill up…, but in English we would usually say The stadium stands fill up… or just The stands fill up….

Leaving out the article, Bancadas do estádio enchem-se…, would sound incomplete or unnatural in Portuguese. For a learner, a good rule is: when you are talking about a specific, identifiable thing (like the stands of a particular stadium), you almost always use the definite article (o, a, os, as).

What is do in do estádio? Why not de o estádio?

Do is a contraction of de + o:

de = of / from
o = the (masculine singular)

So:

de + o → do
do estádio → of the stadium

This contraction is obligatory in normal speech and writing. You would not say de o estádio.

Other similar contractions:

de + a → da (da cidade – of the city)
em + o → no (no estádio – in the stadium)
a + o → ao (ao domingo – on Sunday)

What tense and person is enchem-se?

Enchem-se is:

Verb: encher-se (to fill up, to get full)
Tense: present indicative
Person: 3rd person plural (they)

It matches the subject as bancadas (they):

As bancadas … enchem-se → The stands fill up / get full.

In Portuguese, the present tense often expresses habitual actions, so here it implies something that happens regularly, not just right now.

Why is there a se in enchem-se? Is this really reflexive?

The se here is not truly reflexive (the stands are not doing something to themselves in a conscious way). Instead, this is a pronominal or medio-passive use of the verb:

encher (transitive): encher alguma coisa = to fill something
- Os adeptos enchem as bancadas.
  The fans fill the stands.
encher-se de (pronominal): encher-se de alguma coisa = to become full of something / to fill up with something
- As bancadas enchem-se de adeptos.
  The stands get full of fans.

So encher-se de X roughly means to become full of X.

In European Portuguese, the pronoun usually goes after the verb in the present indicative: enchem-se (not se enchem). The order se enchem is typical of Brazilian Portuguese.

Why is it de adeptos and not com adeptos?

With encher-se, the usual preposition is de, not com:

encher-se de algo → to fill up with / to be filled with something
- enchem-se de adeptos → get full of fans

Using com (encher-se com adeptos) would sound odd or non‑standard here.

You can use com with other verbs though:

Está cheio de adeptos. (also possible: cheio de)
Está com muitos adeptos.

But specifically with encher-se, think de = of.

What does adeptos mean exactly? Is it the same as fans or Brazilian torcedores?

Adepto (plural adeptos) in European Portuguese is the normal word for sports fan / supporter.

In Portugal:
- adeptos de futebol = football fans / supporters
In Brazil, the more common word is torcedores.
English equivalents: fans, supporters.

So:

enchem-se de adeptos ≈ fill up with fans / supporters.

What does ao domingo mean exactly? One specific Sunday or Sundays in general?

Ao domingo here means on Sundays (in general), describing a regular habit.

So the full sentence means something like:

On Sundays, the stadium stands fill up with fans.

If you wanted to talk about one specific Sunday, you would use something like:

no domingo (on that Sunday, often clear from context)
no próximo domingo (next Sunday)
no domingo passado (last Sunday)
este domingo (this Sunday)

Ao domingo is temporal and habitual: it answers When does this usually happen?

Why is it ao domingo and not no domingo?

Ao domingo is:

a + o domingo → ao domingo

It usually expresses habit or routine:

Ao domingo acordo tarde.
On Sundays I wake up late.

By contrast:

no domingo = em + o domingo → more like on (a specific) Sunday, especially when context makes a particular Sunday clear:
- No domingo vamos jogar a final.
  On Sunday we are going to play the final. (this coming Sunday)

Also common for the general/habitual idea:

aos domingos (plural) = on Sundays (habit):
- Aos domingos, o café fecha mais cedo.

So:

ao domingo and aos domingos can both mean on Sundays (habitually).
no domingo tends to point to a specific Sunday.

Can the word order change? For example, can I say Ao domingo, as bancadas do estádio enchem-se de adeptos?

Yes, that word order is perfectly natural:

Ao domingo, as bancadas do estádio enchem-se de adeptos.

You can move the time expression ao domingo around:

Ao domingo, as bancadas do estádio enchem-se de adeptos.
As bancadas do estádio, ao domingo, enchem-se de adeptos.
As bancadas do estádio enchem-se de adeptos ao domingo. (the original)

All are correct; position 1 (putting the time at the beginning) is very common in speech and writing.

Note for European Portuguese:

As bancadas do estádio enchem-se de adeptos. (EP normal)
As bancadas do estádio se enchem de adeptos. (sounds Brazilian)

In European Portuguese present-tense statements with a subject before the verb, the clitic pronoun normally goes after the verb: enchem‑se.

How would this sentence look in Brazilian Portuguese?

A natural Brazilian Portuguese version would be:

As arquibancadas do estádio se enchem de torcedores aos domingos.

Main changes:

bancadas → arquibancadas (more common in Brazil)
adeptos → torcedores
ao domingo → aos domingos (more usual in Brazil)
Pronoun position: se enchem instead of enchem-se

Meaning stays the same: On Sundays, the stadium stands fill up with fans.