Com més aviat reservi les vacances d'agost, menys cars seran els bitllets.

It is a very common Catalan pattern meaning the more ... the less ... or, more generally, the X-er ..., the Y-er ....

So here:

• Com més aviat ... = The earlier ...
• ..., menys cars ... = ..., the less expensive ...

Other examples:

• Com més estudies, més aprens. = The more you study, the more you learn.
• Com menys dorms, pitjor et trobes. = The less you sleep, the worse you feel.

So com in this structure is not really translated as how. It is part of a fixed comparative pattern.

Here més aviat means earlier or sooner.

Literally, it is made up of:

• més = more
• aviat = early / soon

But together, in this kind of sentence, it means earlier:

• com més aviat reservi = the earlier you book

Be careful: més aviat can mean other things in other contexts, such as rather or more precisely, but here the time meaning is the one you need.

Reservi is the present subjunctive form of reservar.

In Catalan, after structures like com més aviat..., especially when talking about a future or hypothetical result, the subjunctive is very common:

• Com més aviat reservi..., menys cars seran...

This is different from English, which usually just uses the present:

• The earlier you book..., the cheaper...

Also, reservi can suggest a formal you (vostè) or a more impersonal whoever/one/you meaning.

Compare:

• reservis = if speaking to tu
• reserveu = if speaking to vosaltres
• reservi = vostè or a general, less personal style

So if you were speaking informally to one person, you might also hear:

• Com més aviat reservis les vacances d'agost, menys cars seran els bitllets.

It can do either, depending on context.

Most likely possibilities:

• formal you: the speaker is addressing one person politely
• general you: like English the earlier you book...
• impersonal/generic: similar to the earlier one books...

Catalan often leaves the subject unstated when it is clear from the verb ending, so there is no explicit vostè or other subject here.

In Catalan, vacances is normally a plural noun, just like in several other European languages.

So:

• les vacances = the holidays / the vacation

Even when English uses singular vacation, Catalan usually uses plural vacances.

Examples:

• Faig vacances al juliol. = I go on vacation in July.
• Les vacances d'agost = the August holidays / the August vacation period

So the plural is normal and expected.

D'agost means of August or in August, depending on how it is translated in context.

It comes from:

• de + agost → d'agost

Catalan shortens de to d' before a vowel. That is why you see the apostrophe.

More examples:

• d'abril = of/in April
• d'estiu = of summer
• d'Europa = of Europe

So les vacances d'agost means the August holidays or the holidays in August.

Menys means less or fewer, and here it modifies the adjective cars.

• car = expensive
• cars = expensive, masculine plural

Since bitllets is masculine plural, the adjective must agree with it:

• el bitllet és car = the ticket is expensive
• els bitllets són cars = the tickets are expensive

So:

• menys cars = less expensive / cheaper

A useful point: menys itself does not change form, but the adjective after it does agree with the noun.

For example:

• menys car with a masculine singular noun
• menys cara with a feminine singular noun
• menys cars with a masculine plural noun
• menys cares with a feminine plural noun

Seran is the future of ser:

• seran = they will be

It agrees with els bitllets:

• els bitllets seran... = the tickets will be...

The sentence is talking about a future consequence:

• if the booking happens earlier, the tickets will be less expensive

So the tense choice is very natural here.

Yes, Catalan could also say:

• Els bitllets seran menys cars.

That word order is perfectly normal.

Putting els bitllets at the end gives a slightly different rhythm and emphasis. The sentence first highlights the result:

• menys cars seran... = less expensive will be...

and then names what is being talked about:

• ...els bitllets = the tickets

Catalan is often more flexible than English with word order, especially when a speaker wants to emphasize a particular part of the sentence.

Not in natural modern English. In Catalan, car means expensive.

So:

• bitllets cars = expensive tickets

Although older or regional English can use dear for prices, the normal translation today is expensive or costly, and in this sentence most learners would naturally think of cheaper for menys cars.