Á þriðjudögum borðum við oft hádegismat saman klukkan ellefu.

Because this sentence describes a repeated habit: on Tuesdays.

A very common Icelandic pattern is:

• á + plural day of the week = on Mondays / on Tuesdays / on Fridays, etc.

So:

• á þriðjudögum = on Tuesdays
• á hverjum þriðjudegi = every Tuesday (more explicit)

A singular form would normally point more toward one specific Tuesday, not a regular habit.

Also, á is the normal preposition used with days of the week in expressions like this.

Because Icelandic main clauses usually follow the verb-second rule.

That means the finite verb tends to come in the second position, even if something other than the subject comes first.

Here, the first element is:

• Á þriðjudögum

So the verb comes next:

• borðum

And then the subject follows:

• við

So:

• Á þriðjudögum borðum við ...

Compare the more neutral subject-first order:

• Við borðum oft hádegismat saman klukkan ellefu.

Both are correct, but the original sentence puts on Tuesdays first for emphasis or topic.

Borðum is the present tense, 1st person plural form of borða (to eat).

So:

• ég borða = I eat
• þú borðar = you eat
• hann/hún/það borðar = he/she/it eats
• við borðum = we eat

In this sentence, borðum við means we eat.

Oft means often, and it is a frequency adverb.

In a basic clause with the subject first, it commonly comes after the subject:

• Við borðum oft hádegismat.

But when another element is moved to the front, Icelandic still keeps the verb in second position:

• Á þriðjudögum borðum við oft hádegismat.

So the order becomes:

1. fronted phrase
2. finite verb
3. subject
4. adverb like oft

This placement sounds natural and standard.

Because hádegismat is the accusative form, and here it is the direct object of borða.

The dictionary form is:

• hádegismatur = lunch

But after borða (to eat), the thing being eaten is in the accusative:

• borða hádegismat = eat lunch

So:

• hádegismatur = nominative
• hádegismat = accusative

This noun is also a compound word:

• hádegi = noon
• matur = food

So it literally means something like midday food.

Because Icelandic does not have an indefinite article like English a/an.

So a bare noun can mean:

• lunch
• a lunch

depending on context.

If you want the lunch, Icelandic usually adds the definite article to the end of the noun:

• hádegismaturinn / hádegismatinn = the lunch
  (the exact form depends on case)

So in this sentence, hádegismat simply means lunch, with no separate word for a.

Saman means together.

It works as an adverb and tells you how the action happens:

• borðum ... saman = eat together

It is very common in sentences about doing something jointly:

• Við vinnum saman = We work together
• Við förum saman = We go together

In your sentence, it naturally connects to the whole action of eating lunch.

Klukkan ellefu is the normal Icelandic way to give a clock time.

• klukkan literally means the clock
• but in time expressions it functions like at ... o’clock

So:

• klukkan ellefu = at eleven o’clock

This is a fixed, everyday pattern:

• klukkan eitt = at one o’clock
• klukkan þrjú = at three o’clock
• klukkan ellefu = at eleven o’clock

So Icelandic does not need a separate at here the way English does.

Yes, Icelandic word order is fairly flexible, but the verb-second rule still matters in main clauses.

For example, these are all possible:

• Á þriðjudögum borðum við oft hádegismat saman klukkan ellefu.
• Við borðum oft hádegismat saman klukkan ellefu á þriðjudögum.
• Klukkan ellefu borðum við oft hádegismat saman á þriðjudögum.

The main difference is focus:

• starting with Á þriðjudögum emphasizes the day
• starting with Klukkan ellefu emphasizes the time
• starting with Við is more neutral

So the sentence can move around, but the finite verb still usually stays in second position.

These two letters are very important in Icelandic:

• þ is pronounced like th in thing
• ð is pronounced like th in this

So in þriðjudögum, you get both sounds.

A few useful notes:

• þ is always unvoiced, like English thin
• ð is voiced, like English then
• the Icelandic sounds are not always exactly identical to English ones, but those comparisons are the best starting point

These two letters often stand out to English speakers, so this is a very common question.