A Ferragosto il lago è pieno di persone.

Here a means on / at the time of. So A Ferragosto means on Ferragosto or during Ferragosto.

This is a common Italian way to refer to holidays or special days:

• A Natale = at Christmas
• A Pasqua = at Easter
• A Ferragosto = on/at Ferragosto

English speakers often expect in or on, but in Italian a is natural here.

Holiday names in Italian often appear without an article when speaking generally.

So:

• A Ferragosto = on Ferragosto

You would not normally say al Ferragosto in this sentence.

Italian commonly uses essere pieno di to mean to be full of.

So:

• Il lago è pieno di persone = The lake is full of people

Saying il lago ha molte persone is grammatically possible in a very literal sense, but it is not the most natural choice here. Essere pieno di is the idiomatic expression.

Pieno is an adjective, and it must agree with the noun it describes.

Here the noun is il lago, which is:

• masculine
• singular

So the adjective must also be masculine singular:

• il lago è pieno

Compare:

• la spiaggia è piena = the beach is full
• i laghi sono pieni = the lakes are full
• le strade sono piene = the streets are full

After pieno, Italian normally uses di before the thing that fills something.

Pattern:

• pieno di + noun

Examples:

• pieno di gente = full of people
• pieno di turisti = full of tourists
• pieno d’acqua = full of water

So pieno di persone is the normal structure.

Both can work, but they are used a little differently.

• persone = people / persons, a regular countable plural noun
• gente = people in a collective sense, grammatically singular in Italian

So you could say:

• Il lago è pieno di persone
• Il lago è pieno di gente

Both are natural. Gente often sounds a bit more general and idiomatic when talking about crowds.

In literal English, the lake is full of people sounds slightly strange because people are not usually in the lake itself. But in Italian, this kind of phrasing can naturally refer to the lake area as a whole, especially in everyday speech.

Depending on context, it may suggest:

• the lakeside is crowded
• the lake area is crowded
• lots of people are around or in the lake

So the sentence is natural even if it is not meant in a strictly literal way.

Yes, absolutely. That would also be very natural.

Compare:

• A Ferragosto il lago è pieno di persone.
• A Ferragosto c’è molta gente al lago.

The first emphasizes that the place is full/crowded.
The second simply says there are a lot of people at the lake.

Italian often puts a time expression first to set the scene.

So:

• A Ferragosto = first, to establish when
• il lago = then the subject
• è pieno di persone = the description

This word order feels very natural in Italian. You could also say:

• Il lago è pieno di persone a Ferragosto

But starting with A Ferragosto gives slightly more emphasis to the time.

Yes. In many contexts, essere pieno di persone does not just mean there are people there; it strongly suggests the place is very busy or crowded.

So the sentence carries the idea:

• the lake is packed
• there are lots of people there
• it gets crowded on Ferragosto

It is pronounced roughly like:

fehr-rah-GOH-stoh

The stress is on go:

• Fer-ra-GO-sto

That stress pattern is important in Italian pronunciation.