På lördag ska vi åka in till centrum och ta en fika.

Swedish normally uses på with days for expressions like on Saturday, on Monday, etc.

• på lördag = on Saturday
• på måndag = on Monday

So på lördag is the natural choice here. It usually refers to the coming Saturday, depending on context.

A useful contrast:

• på lördag = on Saturday / this coming Saturday
• på lördagen = on the Saturday, meaning a more specific Saturday already known from context

This is because Swedish follows the V2 rule in main clauses: the finite verb usually comes in the second position.

In this sentence, På lördag has been placed first. That means the finite verb, ska, must come next:

• På lördag ska vi åka...

If you start with the subject, you get:

• Vi ska åka in till centrum på lördag.

Both are correct. The first version gives a bit more emphasis to Saturday.

Here, ska marks the future, especially a plan, intention, or something that is expected to happen.

So in this sentence, ska is similar to:

• are going to
• will
• sometimes shall, though that sounds less natural in modern English

It does not usually mean obligation here. In other contexts, ska can mean must or is supposed to, but in this sentence it simply shows a future plan.

Swedish distinguishes more clearly than English between different kinds of movement.

• gå = go on foot, walk
• åka = go by some kind of transport, or travel somewhere
• resa = travel, often for longer journeys

So ska vi åka in till centrum suggests going into the center by bus, car, train, etc., or at least thinking of the trip as transportation rather than walking.

If they were definitely walking, Swedish would more naturally use gå:

• På lördag ska vi gå in till centrum...

The in adds the idea of going inward, often toward the town or city center.

So:

• åka till centrum = go to the center
• åka in till centrum = go in to the center / go into town

In many contexts, English would not translate in separately, but it still contributes to the natural sense of movement toward the central area.

In Swedish, places like centrum are often used without an article when talking about going there in a general sense.

• till centrum = to the town center / downtown / into town

Even though the noun is ett centrum, Swedish often drops the article in expressions of destination like this.

Compare:

• Vi åker till centrum. = We’re going to the center / into town.
• Det är ett centrum för forskning. = It is a center for research.

So here, centrum works almost like a fixed destination.

Here, centrum means the town center, city center, or downtown area.

It does not usually mean just any abstract center in this sentence. The context makes it clear that it refers to the central part of town where people go shopping, meet friends, have coffee, and so on.

A very common alternative in everyday Swedish is:

• in till stan = into town

So in till centrum and in till stan can be quite similar, depending on context.

After the modal verb ska, Swedish normally uses the bare infinitive, without att.

So:

• ska åka
• ska ta

not:

• ska att åka
• ska att ta

In this sentence, åka and ta are both infinitives connected by och:

• ska vi åka in till centrum och ta en fika

You can think of it as:

• we are going to [go into town] and [have a fika]

Ta en fika is a very common Swedish expression. Literally it means take a fika, but in natural English it usually means something like:

• have a coffee
• have a coffee break
• go for coffee
• sit down for coffee and something small to eat

A fika is not just the drink. It often means the whole social coffee break: sitting down, chatting, and maybe having a pastry or sandwich.

So ta en fika is an idiomatic everyday expression, and it is extremely common in Swedish.

Yes, fika can be both a noun and a verb.

As a noun:

• en fika = a coffee break / a fika

As a verb:

• att fika = to have a fika / to take a coffee break

So these are both possible:

• Vi ska ta en fika.
• Vi ska fika.

They are very close in meaning. The version with ta en fika is especially common and idiomatic.

Yes. In Swedish, days of the week are normally written with a lowercase letter:

• måndag
• tisdag
• onsdag
• torsdag
• fredag
• lördag
• söndag

So På lördag is correct, not På Lördag.

Yes, absolutely. That is also correct.

The difference is mainly one of focus:

• På lördag ska vi åka in till centrum och ta en fika.
  Emphasis on when.

• Vi ska åka in till centrum och ta en fika på lördag.
  More neutral, with the time placed later.

Both are natural Swedish.

Usually, yes, in ordinary conversation it often means the coming Saturday. But the exact meaning depends on context.

For example:

• If today is Thursday, på lördag will normally mean the Saturday that is coming up.
• In some situations, context may make it clear that another Saturday is meant.

If you want to be more explicit, Swedish can also say:

• nu på lördag = this Saturday / this coming Saturday
• nästa lördag = next Saturday

Those can help avoid ambiguity.