Loppemarkedet støtter gjenbruk og åpner på lørdag.

Because -et is the definite ending for many Norwegian neuter nouns.

• et loppemarked = a flea market
• loppemarkedet = the flea market

So instead of using a separate word like English the, Norwegian often adds the definiteness to the end of the noun.

It literally means flea market:

• loppe = flea
• marked = market

Just like in English, the literal image is not about actual fleas. It refers to a market where people sell used items.

Støtter is the present tense of å støtte = to support.

Norwegian present tense is usually quite simple:

• å støtte = to support
• støtter = support / supports / am supporting / is supporting / are supporting depending on context

Unlike English, Norwegian does not change the verb for different subjects:

• jeg støtter = I support
• du støtter = you support
• loppemarkedet støtter = the flea market supports

Gjenbruk means reuse or re-use.

There is no article because it is being used as an abstract, general idea, similar to English:

• supports reuse
• not necessarily supports the reuse

That is very natural in Norwegian. Abstract nouns often appear without an article when talking about the concept in general.

No, it does not need to be repeated. In Norwegian, just like in English, one subject can belong to two coordinated verbs:

• Loppemarkedet støtter gjenbruk og åpner på lørdag.

This means:

• The flea market supports reuse and opens on Saturday.

The subject loppemarkedet applies to both støtter and åpner.

Åpner is the present tense of å åpne = to open.

So:

• å åpne = to open
• åpner = open / opens

In this sentence, it means opens.

Here it most naturally means opens.

• å åpne = to open
• å være åpen = to be open

So:

• Loppemarkedet åpner på lørdag = The flea market opens on Saturday
• Loppemarkedet er åpent på lørdag = The flea market is open on Saturday

Those are related, but not exactly the same.

På lørdag means on Saturday.

The preposition på is commonly used with days in Norwegian.

Examples:

• på mandag = on Monday
• på fredag = on Friday
• på lørdag = on Saturday

Depending on context, på lørdag can mean:

• on Saturday
• this Saturday
• sometimes next Saturday

Yes, pretty much. Og simply means and and connects the two verbs:

• støtter gjenbruk
• og åpner på lørdag

So the sentence has one subject and two actions.

The infinitives are:

• å støtte = to support
• å åpne = to open

In dictionary form, Norwegian verbs are usually listed with å:

• å støtte
• å åpne

This sentence has normal Norwegian main-clause word order:

• Subject + verb + rest

So:

• Loppemarkedet = subject
• støtter = verb
• gjenbruk og åpner på lørdag = rest of the sentence

Nothing unusual is happening here. It is a straightforward declarative sentence.

Sometimes, but reuse is the better match here.

• gjenbruk = using something again
• resirkulering = recycling

So if the idea is that the flea market promotes using second-hand things again, reuse is the most precise translation.

That would be:

Et loppemarked støtter gjenbruk og åpner på lørdag.

Compare:

• et loppemarked = a flea market
• loppemarkedet = the flea market

So the only change is the noun form.