En el aniversario de su matrimonio, mis tíos preparan una cena sencilla pero muy romántica.

Both are possible, but they say slightly different things:

• En el aniversario de su matrimonio = On the anniversary of their marriage.
  
  • More explicit: it specifies what anniversary (the anniversary of their marriage).
• En su aniversario = On their anniversary.
  
  • Shorter and more general; context would need to make it clear it’s a wedding anniversary.

In Latin American Spanish, you’ll also often hear:

• En su aniversario de bodas (very common: on their wedding anniversary).

In Spanish, su is ambiguous; it can mean:

• his
• her
• their
• your (formal singular or plural, depending on the country)

In this sentence:

En el aniversario de su matrimonio, mis tíos…

Context tells us that su refers to mis tíos (my aunt and uncle), so it means “their”:

• su matrimonio = their marriage / their wedding.

If you needed to avoid ambiguity, you could say:

• …el aniversario del matrimonio de mis tíos
  (the anniversary of my aunt and uncle’s marriage)

Both relate to getting married, but they focus on different aspects:

• matrimonio = marriage as a state/relationship; also used for married couple.
  
  • aniversario de su matrimonio = the anniversary of their marriage.
• boda = the wedding ceremony/party itself.
  
  • aniversario de bodas (very idiomatic) = wedding anniversary.

So:

• aniversario de su matrimonio
• aniversario de bodas

are both understood as wedding anniversary, though aniversario de bodas may sound a bit more set‑phrase-like.

• en focuses on time: “on” that date → on their anniversary.
• para would mean for, in the sense of for the purpose of / in preparation for.

So:

• En el aniversario de su matrimonio, mis tíos preparan…
  = On the anniversary of their marriage, my aunt and uncle prepare… (when they do it)

If you said:

• Para el aniversario de su matrimonio, mis tíos preparan una cena…
  it would sound like: For their anniversary, my aunt and uncle prepare a dinner… (in preparation for that event). It’s possible, but the nuance is different.

Spanish often uses the simple present for:

• habitual actions (things they do regularly):
  • Mis tíos preparan una cena…
    = My aunt and uncle prepare / usually prepare a dinner…
• general truths, routines, etc.

So preparan here very naturally means they (typically) prepare a dinner on each anniversary.

If you wanted to emphasize just one future occasion, you could say:

• …mis tíos van a preparar una cena… (are going to prepare / will prepare)

In Spanish, subject pronouns (yo, tú, él, ella, ellos, etc.) are usually omitted because the verb ending already tells you the subject.

• Mis tíos preparan… = clear subject from the noun phrase.
• Ellos preparan… would also be grammatical, but:
  • you usually only add ellos for contrast or emphasis (e.g. they do, but others don’t).

“Mis tíos ellos preparan…” sounds redundant or wrong in standard Spanish; you normally pick either the noun phrase (mis tíos) or the pronoun (ellos).

Mis tíos can mean:

• my uncles, or
• my aunt and uncle (one male + one female)
• my aunts and uncles (mixed group)

Spanish uses the masculine plural (tíos) for any mixed group. Here, because we’re talking about their marriage and a romantic dinner, we naturally imagine a married couple: an aunt and an uncle.

In Spanish, the default position of descriptive adjectives is after the noun:

• una cena sencilla = a simple dinner
• una cena romántica = a romantic dinner

Putting the adjectives before the noun (una sencilla cena, una romántica cena) is possible but:

• Sounds more literary, poetic, or emphatic.
• Can sometimes change nuance or sound unnatural in everyday speech.

So the natural order in normal conversation is:

una cena sencilla pero muy romántica

Both exist in Spanish, but they’re used slightly differently:

• sencilla (from sencillo/a)
  • Common for things that are modest, not fancy, uncomplicated.
  • una cena sencilla = modest, not elaborate, but positive in tone.
• simple can work similarly, but:
  • In some contexts, it can feel more neutral or even a bit negative (like “plain” or “basic”).
  • Una cena simple is okay, but cena sencilla is more idiomatic and warm.

So “una cena sencilla” sounds like a nice, modest dinner, not cheap or disappointing.

Adjectives in Spanish must agree with the noun in gender and number.

• cena is a feminine singular noun:
  • la cena
• So the adjectives must also be feminine singular:
  • sencilla, not sencillo
  • romántica, not romántico

If the noun were masculine plural, for example:

• los platos son sencillos pero muy románticos
  (the dishes are simple but very romantic)
  → adjectives change to -os.

You can absolutely say:

• una cena sencilla pero romántica = a simple but romantic dinner

Adding muy:

• una cena sencilla pero muy romántica
  makes it stronger: very romantic.

So muy functions just like English “very”, intensifying the adjective:

• romántica = romantic
• muy romántica = very romantic

Grammatically, versions like:

• una muy romántica y sencilla cena

are possible, but:

• They sound literary, poetic, or stylistically marked.
• They are not the neutral, everyday way to say this.

In normal spoken or written Spanish, the most natural version is exactly:

una cena sencilla pero muy romántica

Noun first, then adjectives.

Yes, but it changes the meaning:

• la cena = the dinner (specific, probably the main evening meal that day).
• una cena sencilla pero muy romántica = a (particular kind of) dinner described as simple but very romantic.

So:

• …preparan la cena → They just make dinner (no description).
• …preparan una cena sencilla pero muy romántica → They prepare a special dinner that is simple but very romantic.

You choose depending on how much detail or emphasis you want to give to the type of dinner.