Dezembro é o mês de que quase todo mundo gosta por causa das festas.

In Brazilian Portuguese, names of months usually appear without an article in neutral, dictionary-style usage:

• Dezembro é...
• Janeiro foi quente.
• Outubro chega logo.

That is very normal.

In some varieties of Portuguese, especially in European Portuguese and in some regional or stylistic uses, you may sometimes hear an article with months, but for Brazilian Portuguese, no article is the safest default for learners in sentences like this.

Because Portuguese normally uses the definite article here when identifying something as a specific category:

• Dezembro é o mês... = December is the month...
• Pelé foi o jogador... = Pelé was the player...
• Esse é o livro... = This is the book...

Saying Dezembro é mês... would sound incomplete or unnatural in this context.

This happens because the verb gostar requires the preposition de:

• gostar de algo
• gostar de alguém

So if you turn o mês into the thing that people like, you must keep that de in the relative clause:

• o mês de que quase todo mundo gosta

Literally, the structure is something like:

• the month that almost everyone likes
• but in Portuguese grammar: the month of which almost everyone likes

That sounds strange in English, but it is normal in Portuguese because gostar needs de.

In careful standard Portuguese, no. The sentence needs the preposition required by gostar de.

So the standard forms are:

• o mês de que quase todo mundo gosta
• o mês do qual quase todo mundo gosta

You may hear people omit the preposition in casual speech, but learners should treat that as nonstandard for formal writing and for grammar-based study.

In this sentence, they mean the same thing:

• o mês de que quase todo mundo gosta
• o mês do qual quase todo mundo gosta

Both are correct.

Usually:

• de que is more common and lighter
• do qual / da qual / dos quais / das quais sounds a bit more formal or heavier

Since mês is masculine singular, do qual matches it.

Because todo mundo is grammatically singular in Portuguese, even though its meaning is plural-like.

So you say:

• todo mundo gosta
• quase todo mundo gosta
• todo mundo sabe

Not:

• todo mundo gostam
• quase todo mundo gostam

This is similar to English everyone likes, not everyone like.

Quase todo mundo means almost everyone.

Breakdown:

• quase = almost
• todo mundo = everyone / everybody

So:

• todo mundo gosta = everyone likes
• quase todo mundo gosta = almost everyone likes

Because por causa de is used before a noun phrase, while porque introduces a clause.

Here we have a noun phrase:

• das festas = of the celebrations / parties / holidays

So:

• por causa das festas = because of the celebrations

Compare:

• Quase todo mundo gosta de dezembro por causa das festas.
• Quase todo mundo gosta de dezembro porque há muitas festas.

Both are correct, but they are built differently.

Because das is the contraction of de + as.

• de + as = das

This happens because por causa de is followed by as festas:

• por causa de as festas → por causa das festas

This kind of contraction is very common in Portuguese:

• de + o = do
• de + a = da
• de + os = dos
• de + as = das

Here festas probably refers to the holiday celebrations associated with December in Brazil, especially things like:

• Christmas celebrations
• New Year’s celebrations
• end-of-year gatherings
• family parties
• company parties

So it does not necessarily mean only formal parties. It can mean celebrations / festivities more broadly.

Yes, it is grammatical and natural, but it is a bit more structured and descriptive than everyday conversation.

A more direct everyday version might be:

• Quase todo mundo gosta de dezembro por causa das festas.

That version starts with the people and the action.
The original sentence:

• Dezembro é o mês de que quase todo mundo gosta por causa das festas.

puts more focus on December and describes it as the month that almost everyone likes.

So both are fine, but the original has a slightly more written or emphatic feel.

É is the 3rd person singular present tense of ser:

• eu sou
• você é
• ele/ela é

Since the subject is Dezembro, the sentence uses é:

• Dezembro é o mês...

This is the normal way to say December is...

Yes. This is a very important Portuguese pattern.

When the verb in the relative clause requires a preposition, the preposition must stay there.

Examples:

• o livro de que preciso
  because precisar de
• a pessoa em quem confio
  because confiar em
• a cidade de que me lembro
  because lembrar-se de
• o assunto sobre que falamos / sobre o qual falamos
  because falar sobre

So in your sentence:

• gostar de → o mês de que quase todo mundo gosta

This is one of the most useful grammar patterns to notice in Portuguese.