Em dezembro, o Pedro diz que prefere descansar em casa e planear o próximo ano.

Because Portuguese normally uses em with months to mean in:

• em dezembro = in December
• em janeiro = in January

You would not normally say no dezembro here. With months, Portuguese usually does not use an article in this kind of time expression.

In Portuguese, months are normally written with a lower-case letter, unlike in English.

So:

• dezembro = correct
• Dezembro = only if it begins the sentence or appears in a title/style that requires capitalization

The same is true for days of the week and seasons.

The comma separates the opening time expression from the rest of the sentence:

• Em dezembro, o Pedro diz...

This is very common when a sentence starts with a time phrase, place phrase, or other introductory element. It helps readability. In some short sentences the comma may be omitted, but here it is perfectly natural.

In European Portuguese, it is very common to use the definite article before a person's first name:

• o Pedro
• a Maria

This sounds natural in everyday Portuguese from Portugal. English does not do this, so it often feels strange to learners.

A few notes:

• It is especially common in speech and informal writing.
• In very formal writing, the article may be omitted more often.
• This is much more typical in Portugal than in standard Brazilian Portuguese.

After a verb like dizer (to say), Portuguese normally introduces the next clause with que:

• diz que prefere... = says that he prefers...

Here, que works like English that.
In English, that is often omitted:

• He says (that) he prefers...

In Portuguese, que is usually kept.

Because when the same person does both actions, Portuguese usually uses the infinitive after preferir:

• O Pedro prefere descansar...
• O Pedro prefere planear...

So:

• prefere descansar em casa e planear o próximo ano

means that Pedro prefers to rest and to plan.

This is very similar to English prefer to do something, although Portuguese often uses the bare infinitive without a separate word for to.

Because both infinitives depend on prefere, and both have the same subject: o Pedro.

So this structure means:

• he prefers [descansar em casa] and [planear o próximo ano]

Portuguese does not need to repeat the subject if it is clearly the same person doing both actions.

Em casa is the normal expression for at home or home in a general sense.

• descansar em casa = rest at home

But na casa usually means in the house or at the house, often referring to a specific house:

• na casa do Pedro = at Pedro's house
• estou na casa da minha irmã = I am at my sister's house

So:

• em casa = at home
• na casa = in/at the house

Yes. Planear is the standard European Portuguese verb meaning to plan.

So:

• planear o próximo ano = to plan the next year / to plan for the coming year

A useful comparison:

• European Portuguese: planear
• Brazilian Portuguese: planejar is much more common

So this sentence clearly looks like Portuguese from Portugal.

In Portuguese, a noun phrase like this normally takes the definite article:

• o próximo ano = the next year / next year

Portuguese uses articles more often than English does, so where English says next year, Portuguese often says o próximo ano.

Without the article, próximo ano can appear in very compressed styles such as notes, headings, or certain fixed contexts, but in a normal full sentence o próximo ano is the most natural choice here.

Yes. Both are present indicative forms:

• diz = he says
• prefere = he prefers

This can describe:

• what Pedro says now, or
• a habitual/general situation, depending on context

Portuguese uses the present tense in much the same way English does here.

Yes. Portuguese is flexible with word order, especially with time expressions. For example:

• Em dezembro, o Pedro diz que prefere descansar em casa...
• O Pedro diz, em dezembro, que prefere descansar em casa...
• O Pedro diz que, em dezembro, prefere descansar em casa...

The original version is very natural because it puts the time frame first. That helps set the scene immediately: In December...

Preferir usually implies a preference in relation to some alternative, even if that alternative is not stated.

So this sentence suggests that Pedro prefers:

• resting at home, and
• planning the next year

rather than doing something else.

Portuguese often leaves the comparison unstated if it is obvious from context.