Vi reser till Spanien den fjärde januari.

In Swedish, the present tense is very often used for the future when the time is clear from the context.

Here, den fjärde januari makes it obvious that the trip will happen in the future, so Vi reser till Spanien den fjärde januari sounds completely natural.

Swedish can also use ska for the future:

• Vi ska resa till Spanien den fjärde januari.

That can sound a little more like a plan or intention, but in many cases both versions work.

Yes, you often could say åker here.

• resa = to travel, to make a journey
• åka = to go/travel, often especially by vehicle

So:

• Vi reser till Spanien den fjärde januari.
• Vi åker till Spanien den fjärde januari.

Both are possible, but they feel slightly different:

• reser is a bit more neutral or formal, focusing on the trip itself
• åker is very common in everyday speech

An English speaker can think of resa as closer to travel, while åka is often closer to go.

till means to, and it is used here because the sentence describes movement toward a destination.

• till Spanien = to Spain

Compare:

• Vi reser till Spanien. = We are traveling to Spain.
• Vi är i Spanien. = We are in Spain.

So the difference is:

• till = movement toward a place
• i = being inside/in a place

Spanien is simply the Swedish name for Spain. Country names in Swedish are usually used without an article.

So:

• Spanien = Spain
• Italien = Italy
• Sverige = Sweden

You normally just say:

• Vi reser till Spanien.

not something like till det Spanien or till Spanienet.

So in this sentence, Spanien works as a proper noun, just like Spain in English.

That is a standard Swedish way to say a date:

• den + ordinal number + month

So:

• den fjärde januari = the fourth of January / January fourth

Some more examples:

• den första maj = the first of May
• den tionde april = the tenth of April

This is one of the most useful date patterns to learn in Swedish.

In dates, den is part of the normal full expression.

It works a bit like the in English date expressions such as the fourth of January.

So:

• den fjärde januari = the fourth of January

In short written forms, Swedish often leaves den out:

• 4 januari
• 4/1

But in a fully spoken-out date, den is very common and natural.

Because dates use ordinal numbers, not cardinal numbers.

• fyra = four
• fjärde = fourth

English does the same:

• four vs fourth

So in a date, Swedish uses fjärde, because you are saying the fourth day of the month, not just the number four.

Also, fjärde is irregular, so it does not look exactly like fyra.

In Swedish, months are not capitalized, unless they begin a sentence.

So Swedish writes:

• januari
• februari
• mars

This is different from English, where January must be capitalized.

So Spanien has a capital letter because it is a country name, but januari stays lowercase.

Yes. Swedish can move the time expression to the front:

• Den fjärde januari reser vi till Spanien.

This means the same thing, but it puts more focus on the date.

Notice the word order:

• Vi reser till Spanien den fjärde januari.
• Den fjärde januari reser vi till Spanien.

In the second sentence, reser still stays in the second position. That is a very important Swedish word-order rule in main clauses.