Nelle giornate piovose, preferisco rimanere in casa.

Nelle is the contracted form of in + le.

• in = in / on / during
• le = the (feminine plural)

So:

• nelle giornate = in the days / on the days

Italian often combines prepositions with articles:

• in + il = nel
• in + lo = nello
• in + la = nella
• in + i = nei
• in + gli = negli
• in + le = nelle

So in le giornate would be incorrect in standard Italian.

Italian often uses the plural to talk about something that happens generally or habitually.

So nelle giornate piovose means something like:

• on rainy days
• during rainy days
• whenever the days are rainy

Using the plural makes it sound like a general preference, not just one specific rainy day.

Both giorno and giornata can mean day, but they are not always used in exactly the same way.

• giorno often refers more neutrally to a day as a unit of time
• giornata often suggests the day as it is experienced, especially in descriptions like weather, mood, or how the day goes

That is why Italian very naturally says:

• una bella giornata
• una giornata piovosa
• una brutta giornata

So nelle giornate piovose sounds very natural when talking about weather conditions.

In Italian, adjectives often come after the noun, especially when they are describing a factual quality.

So:

• giornate piovose = rainy days

This is the normal order here.
Also, piovose must agree with giornate:

• giornata = feminine singular
• giornate = feminine plural

So the adjective becomes feminine plural too:

• piovosa → piovose

Piovose means rainy.

It comes from piovoso, an adjective related to rain.
It describes days that have a lot of rain or rainy weather.

You may also hear expressions with di pioggia, but giornate piovose is a very natural and direct way to say rainy days.

Examples:

• una settimana piovosa = a rainy week
• un clima piovoso = a rainy climate

Because preferire is commonly followed by an infinitive when you say you prefer to do something.

So:

• preferisco rimanere = I prefer to stay

This is the same basic structure as:

• preferisco leggere = I prefer to read
• preferisco uscire la sera = I prefer to go out in the evening

Italian does not need a separate word for to before the infinitive in this structure.

Both rimanere and restare can mean to stay or to remain, and in many contexts they are interchangeable.

So you could also say:

• preferisco restare in casa

Both are natural.

Very broadly:

• rimanere can sound slightly more like remain
• restare can sound slightly more like stay

But in everyday Italian, the difference is often small. A learner can usually understand both as valid here.

In casa is a fixed, very common expression meaning:

• at home
• indoors
• in the house

Italian often drops the article in this expression.

So:

• stare in casa
• rimanere in casa
• restare in casa

are all very natural.

If you said nella casa, that would usually mean in the house in a more specific sense, referring to a particular house.

Yes, often you can, but there is a slight nuance.

• a casa usually means at home
• in casa often emphasizes inside the house / indoors

In this sentence, both are possible:

• preferisco rimanere in casa
• preferisco rimanere a casa

The original version with in casa gives a stronger feeling of staying indoors because of the rainy weather.

The comma separates the introductory time phrase from the main clause.

• Nelle giornate piovose, = On rainy days,
• preferisco rimanere in casa. = I prefer to stay at home / indoors.

In Italian, this comma is common and helps readability, especially when a phrase is placed at the beginning for emphasis or context.

You may sometimes see similar sentences without a comma, but with this kind of opening phrase, the comma is very normal.

Yes. Italian word order is flexible.

The sentence could also be:

• Preferisco rimanere in casa nelle giornate piovose.

This means the same thing.
The difference is mainly emphasis:

• Nelle giornate piovose, preferisco rimanere in casa. puts the focus first on the situation: rainy days
• Preferisco rimanere in casa nelle giornate piovose. starts with the main idea: I prefer to stay in

Both are correct and natural.

It is a general statement.

Several things show that:

• nelle giornate piovose is plural and generic
• preferisco is in the present tense, which often expresses habits, preferences, and general truths

So the sentence means this is the speaker’s usual preference whenever the weather is rainy.

Yes, and the meaning would be very similar.

• Nelle giornate piovose, preferisco rimanere in casa.
• Quando piove, preferisco rimanere in casa.

Both are natural.

The difference is slight:

• nelle giornate piovose sounds a bit more descriptive and stylistically polished
• quando piove is a bit more direct and conversational

So a learner should understand them as close equivalents in many contexts.