Dimarts, si arribes aviat, et guardaré una butaca al costat de la meva.

Yes. In Catalan, days of the week often appear without a preposition when they mean on that day.

• Dimarts = Tuesday / on Tuesday, depending on context
• So Dimarts, si arribes aviat... means On Tuesday, if you arrive early...

This is very natural in Catalan.

Because Catalan normally uses the present indicative after si for a real future condition.

So:

• si arribes aviat = if you arrive early
• et guardaré... = I’ll save you...

This is the standard pattern:

• si + present
• future in the main clause

So si arribaràs would sound wrong here in standard Catalan.

Arribes is the 2nd person singular present indicative of arribar (to arrive).

Conjugation:

• jo arribo
• tu arribes
• ell/ella arriba
• nosaltres arribem
• vosaltres arribeu
• ells/elles arriben

So si arribes aviat means if you arrive early.

It can mean soon, early, or before long, depending on context.

In this sentence, because it goes with arribes (you arrive), the most natural meaning is:

• arribes aviat = you arrive early

So here it is best understood as early.

Et is the indirect object pronoun meaning to you or for you.

So:

• guardaré una butaca = I’ll save a seat
• et guardaré una butaca = I’ll save you a seat

Catalan often uses these short object pronouns before the verb.

Some related forms are:

• em = to/for me
• et = to/for you
• li = to/for him/her
• ens = to/for us
• us = to/for you all
• els = to/for them

Because the speaker is talking about something they will do later.

Guardaré is the 1st person singular future of guardar:

• infinitive: guardar
• future: guardaré = I will save / I’ll keep / I’ll reserve

Here it means something like:

• I’ll save
• I’ll keep
• I’ll reserve

depending on the situation.

Literally, guardar often means to keep, to store, or to put away. But in this context, it means:

• to save
• to keep aside
• to reserve

So et guardaré una butaca means I’ll save you a seat.

This is a common and natural use of guardar in Catalan.

Butaca usually refers to a seat, often a more substantial or cushioned one, such as in:

• a cinema
• a theatre
• an auditorium

It can also mean armchair in other contexts.

By contrast:

• cadira = chair

So here butaca is best translated as seat.

Al costat de means next to or beside.

So:

• al = contraction of a + el
• costat = side
• de = of
• la meva = mine / my one

Together:

• al costat de la meva = next to mine

It is short for something like:

• al costat de la meva butaca

The noun butaca is omitted because it is obvious from the context.

In standard Catalan, possessives usually take an article when they stand with a noun or replace one.

So you normally get:

• la meva butaca = my seat
• la meva = mine

That is why the sentence says de la meva, not just de meva.

The commas help separate the different parts of the sentence:

1. Dimarts is an introductory time expression.
2. si arribes aviat is a conditional clause.
3. et guardaré una butaca al costat de la meva is the main clause.

So the commas make the structure clearer:

• Dimarts, = On Tuesday,
• si arribes aviat, = if you arrive early,
• et guardaré... = I’ll save you...

In writing, this punctuation is very natural.

Yes, but et guardaré una butaca is the most natural and compact way.

You could say something like:

• guardaré una butaca per a tu

but Catalan very often prefers the object pronoun:

• et guardaré una butaca

That is the normal everyday phrasing.