Mis amigos quieren protestar en la manifestación del sábado.

In Spanish you normally don’t combine a possessive adjective (mi, tu, su, nuestro, vuestro, su) with a definite article (el, la, los, las) in front of the noun.

• Mis amigos = my friends (standard way)
• Los amigos = the friends
• Los mis amigos ❌ sounds wrong in modern standard Spanish.

So you either say mis amigos (to express possession) or los amigos (just “the friends”), but not both together.

Amigos can be:

• a group of only male friends, or
• a mixed group of male and female friends.

Spanish uses the masculine plural form (amigos) when the group is mixed or when gender is unknown.
If the group is only women, you’d say mis amigas.

Because the subject is the same for both verbs.

• Subject: mis amigos
• Action 1: quieren (they want)
• Action 2: protestar (they protest)

In Spanish, when both actions have the same subject, you usually use querer + infinitive:

• Mis amigos quieren protestar. = My friends want to protest.

You use querer que + subjunctive when the second action has a different subject:

• Mis amigos quieren que yo proteste. = My friends want me to protest.
• Mis amigos quieren que nosotros protestemos. = My friends want us to protest.

After verbs like querer, poder, deber, necesitar, etc., Spanish normally uses the infinitive to express what the subject wants/can/has to do:

• Quieren protestar. = They want to protest.
• Pueden venir. = They can come.
• Debemos estudiar. = We must study.

So protestar is in the infinitive because it depends on quieren.

The preposition depends on what you want to express:

• ir a la manifestación = to go to the demonstration (movement towards a place → a)
• protestar en la manifestación = to protest at the demonstration (the place where the action happens → en)

So:

• Movement: a
• Location where something happens: en

That’s why it’s protestar en la manifestación.

That would change the meaning.

• protestar en la manifestación = protest at the demonstration (you are part of that demonstration).
• protestar contra la manifestación = protest against the demonstration itself (you oppose that demonstration).

So en = the event is the place where you protest;
contra = the event or thing is the target of your protest.

Both are possible, but the nuance is slightly different.

1. la manifestación del sábado

   • Literally: the Saturday demonstration / Saturday’s demonstration
   • del = de + el → “of the”
   • It treats sábado as a characteristic of the demonstration, as if there are several demonstrations and you’re identifying “the one on Saturday.”

2. la manifestación el sábado

   • Here el sábado works more like a time expression:
     
     • the demonstration (which is) on Saturday
   • Example sentence: Mis amigos quieren protestar en la manifestación el sábado.

In your sentence, la manifestación del sábado sounds like a specific, already-known event: the demonstration on Saturday that everyone is talking about.

Yes, that’s also correct:

• Mis amigos quieren protestar el sábado en la manifestación.

Here the structure is:

• Time: el sábado
• Place: en la manifestación

This version sounds a bit more like neutral “time + place” information.
The original la manifestación del sábado packs both ideas into one noun phrase and suggests we’re talking about “that particular Saturday demonstration.” Both are natural in Spain; which one you choose depends on what you want to emphasize.

Using la (the definite article) implies that both speaker and listener know which demonstration is meant:

• la manifestación del sábado = the Saturday demonstration (known, specific event)

If you said:

• Mis amigos quieren protestar en una manifestación del sábado.

this would sound like:

• My friends want to protest at *a demonstration on Saturday* (one among several, more vague).

So la makes it clear we’re referring to a specific, identified event.

In Spain:

• manifestación usually means a public demonstration or march, often organized, with people walking in the streets, banners, etc.
• protesta is more general and can be:
  • a spoken complaint,
  • a small act of protest,
  • or a bigger movement.

You can say:

• Ir a una manifestación = go to a (street) demonstration
• Hacer una protesta / una protesta simbólica = carry out some kind of protest

In this context, manifestación is the standard word for a political or social march in the streets.

Yes, but they’re related.

• protestar = to protest (general verb)

  • Mis amigos quieren protestar. = My friends want to protest.

• manifestarse (reflexive) = literally to demonstrate (oneself), meaning to take part in a demonstration / march.

  • Mis amigos quieren manifestarse el sábado. = My friends want to join/take part in the demonstration on Saturday.

You can easily hear in Spain:

• Vamos a manifestarnos el sábado. = Let’s go and join the demo on Saturday.

In your sentence protestar en la manifestación focuses on the action of protesting, happening at the demonstration. Manifestarse focuses more on joining the march itself. Both are natural.

Spanish accents mark the stressed syllable when it doesn’t follow the regular stress rules.

• manifestación

  • Ends in -n, so the natural stress would be on the second-to-last syllable: mani-fes-ta-*CIÓN? (wrong)
  • But the real stress is on the last syllable: mani-fes-ta-CIÓN
  • Because that’s irregular, it needs an accent: manifestación.

• amigos

  • Ends in -s, so the stress falls naturally on the second-to-last syllable: a-MI-gos.
  • That matches the rule → no accent.

• quieren

  • Ends in -n, so stress on second-to-last syllable: QUIE-ren.
  • Also matches the rule → no accent.

So the accent in manifestación is just to show where the stress goes.

Querer can mean both:

1. to want

   • Quiero comer. = I want to eat.
   • Mis amigos quieren protestar. = My friends want to protest.

2. to love (especially with people, sometimes with things)

   • Quiero a mis amigos. = I love my friends.
   • Te quiero. = I love you.

The meaning is clear from the structure:

• querer + infinitive (like protestar) → to want to do something
• querer + a + person / thing → usually to love / care for

In your sentence, quieren protestar can only mean want to protest, not “love to protest.”