Novembro é o mês em que eu começo a pensar no final do ano.

Normally, no. In Portuguese, month names are usually written in lowercase: novembro, dezembro, janeiro, etc.

In this sentence, Novembro is capitalized only because it is the first word of the sentence.

Portuguese often uses month names without an article when speaking generally:

• Novembro é um mês importante.
• Dezembro chega logo depois.

In Brazilian Portuguese, you can sometimes hear articles with months in certain contexts, but in a sentence like this, novembro without an article sounds perfectly natural and standard.

Because o mês means the month, and the sentence is identifying November as a specific month in relation to the speaker’s habit:

• Novembro é o mês... = November is the month...

If you said é mês, it would sound incomplete or unnatural here. Portuguese, like English, often uses the definite article when defining something in this way.

Em que here means in which or, more naturally in English, when.

So:

• o mês em que eu começo...
• literally: the month in which I begin...
• naturally: the month when I begin...

This is a very common structure in Portuguese after nouns of time:

• o dia em que = the day when
• a época em que = the time/period when
• o momento em que = the moment when

Sometimes, yes, but em que is the safest and most standard choice here.

• Novembro é o mês em que eu começo... = very natural and standard
• Novembro é o mês quando eu começo... = some people may say this, but it is less standard

For learners, it is best to use em que after a noun like mês.

Yes, it can be omitted.

Portuguese verbs often show who the subject is, so both are possible:

• ...em que eu começo a pensar...
• ...em que começo a pensar...

Both mean the same thing. Including eu can add clarity or a slight sense of emphasis, but it is not required here.

This is the verb começar followed by a + infinitive:

• começar a pensar = to begin/start to think

This is a very common Portuguese pattern:

• começar a estudar = to start studying
• começar a trabalhar = to start working
• começar a chover = to start raining

So eu começo a pensar means I begin to think or I start thinking.

Because the sentence describes a habitual or recurring action.

It means something like:

• November is the month when I start thinking about the end of the year
• in the sense of this is what usually happens to me every year

Portuguese often uses the present tense for habits, routines, and general truths, just like English does.

Because the verb pensar usually needs the preposition em when it means to think about:

• pensar em alguma coisa = to think about something

So:

• pensar em o final do ano becomes
• pensar no final do ano

That is because em + o = no.

This contraction is extremely common in Portuguese.

No is a contraction of:

• em + o = no

So:

• no final = in the end / at the end

The same happens with do:

• de + o = do

So:

• do ano = of the year

Altogether:

• no final do ano = at the end of the year

Pensar em is the most natural choice for think about in this sentence.

• pensar em often means to have something in mind, to think about, or to start considering
• pensar sobre can sound more reflective, analytical, or formal, like to reflect on or to think over

Here, começo a pensar no final do ano sounds very natural because it suggests that the end of the year starts entering your thoughts.

In many cases, they are very similar and both can mean the end of the year.

• no final do ano
• no fim do ano

Both are common in Brazilian Portuguese.

A very rough nuance is:

• final can sound a little more neutral or slightly more formal
• fim can sound a little more direct or everyday

But in this sentence, either one would be understood naturally.

Yes. A very natural alternative is:

• Em novembro, eu começo a pensar no final do ano.

This means almost the same thing: In November, I start thinking about the end of the year.

The difference is mainly focus:

• Novembro é o mês em que... emphasizes November as the month
• Em novembro, eu começo... emphasizes what happens in November

Both are correct and natural.