Julho é o mês de que meus filhos mais gostam, por causa das férias.

Yes. In Portuguese, month names are normally written in lowercase: julho, agosto, setembro, etc.

Here it is capitalized only because it is the first word of the sentence. In the middle of a sentence, you would usually write julho.

A very literal breakdown is:

• Julho = July
• é = is
• o mês = the month
• de que = that / of which
• meus filhos = my children
• mais gostam = like the most
• por causa das férias = because of the vacation / holidays

So the structure is roughly:

July is the month that my children like the most, because of the vacation.

The Portuguese word order is a little different from natural English, but the idea is straightforward.

Because the verb gostar normally requires the preposition de:

• gostar de algo = to like something

So:

• Meus filhos gostam de julho = My children like July

When that idea becomes a relative clause, the de must stay:

• o mês de que meus filhos gostam = the month that my children like

So de que is there because gostar needs de.

In standard grammar, no. The verb gostar needs de, so the more correct form is:

• o mês de que meus filhos mais gostam

You may hear some native speakers drop the preposition in casual speech, but learners should keep de here.

A good rule is:

• if the verb needs a preposition, the relative structure usually keeps it.

Because the de has already been placed before que.

Compare:

• Meus filhos gostam de julho.
• Julho é o mês de que meus filhos gostam.

That de still belongs to gostar, but in the relative clause it moves in front of que. So you do not say:

• de que meus filhos gostam de ❌

Because mais is modifying the idea of liking. It means most here.

• gostam = they like
• mais gostam = they like the most

So:

• o mês de que meus filhos mais gostam = the month my children like most

In Portuguese, words like mais often come before the verb in this kind of structure.

Both are possible.

In Brazilian Portuguese, possessives can appear:

• without the article: meus filhos
• with the article: os meus filhos

So both of these are natural:

• meus filhos
• os meus filhos

The version without the article is perfectly normal. The version with the article can sound a little more specific or just reflect personal/regional preference.

It means because of the vacation or because of the holidays.

Breakdown:

• por causa de = because of
• as férias = the vacation / the holidays
• de + as = das

So:

• por causa das férias = because of the vacation/holidays

Because férias is normally a plural noun in Portuguese, even when it refers to one vacation period.

So Portuguese says:

• as férias

not usually a singular féria with this meaning.

This is similar to how some concepts are plural in one language but singular in another.

Yes. You could say:

• Julho é o mês do qual meus filhos mais gostam.

This is grammatically correct, but it sounds more formal and heavier. In everyday Portuguese, de que is usually more natural here.

So:

• de que = common and natural
• do qual = correct, but more formal/stiff

Not always. The sentence works with or without it:

• Julho é o mês de que meus filhos mais gostam, por causa das férias.
• Julho é o mês de que meus filhos mais gostam por causa das férias.

The comma gives a slight pause and makes the reason sound a bit more like an added explanation. Without the comma, the sentence feels a little tighter and more direct.

Yes. A very natural alternative would be:

• Julho é o mês favorito dos meus filhos por causa das férias.

That means basically the same thing: July is my children’s favorite month because of vacation.

The original sentence is correct, but this alternative may feel simpler to many learners because it avoids the relative-clause structure de que ... gostam.