Learn Hardcore Catalan: A la primavera, el mar és fred.

You may also hear en primavera, which is also possible in many contexts. But a la primavera is a very normal, natural choice.
What is much less natural here is en la primavera, unless you are talking about a specific spring already identified.

Why is there an article in la primavera?

Catalan often uses the definite article with seasons and many other nouns where English does not.

So English in spring becomes Catalan a la primavera.

This is very normal in Catalan grammar and not something unusual about this sentence.

Why does it say el mar instead of just mar?

Catalan usually uses the definite article when talking about something in a general sense.

So:

el mar = the sea, including the sea in general
not necessarily one specific sea already mentioned

English often drops the article in more places than Catalan does, but Catalan prefers it here.

Why el mar and not la mar?

Both el mar and la mar exist in Catalan.

el mar is the most neutral and standard form
la mar can sound more literary, poetic, or regional

So in a basic sentence like this, el mar is the expected choice.

Why is és written with an accent?

És is the he/she/it is form of the verb ser.

The accent is important because it distinguishes és from es:

és = is
es = a pronoun, as in reflexive structures

So the accent is not optional here; it changes the word.

Why is the verb ser used here instead of estar?

Here és fred presents the sea as generally cold in spring, as a general characteristic or usual condition.

In Catalan, ser is commonly used for this kind of general statement.

Estar can be used in Catalan too, but it usually sounds more like a temporary or immediate state in a particular situation. So:

El mar és fred a la primavera = the sea is cold in spring, generally
El mar està fred = the sea is cold right now / at the moment / in its current state

That is the main difference a learner should notice here.

Why is it fred and not freda?

Because adjectives in Catalan agree with the noun they describe.

Here the noun is:

el mar = masculine singular

So the adjective must also be masculine singular:

fred = cold

If the noun were feminine singular, you would normally use freda instead.

Is the comma necessary in A la primavera, el mar és fred?

The comma is helpful, but not absolutely necessary.

It separates the introductory time phrase A la primavera from the rest of the sentence. This is very natural in writing:

A la primavera, el mar és fred.

You may also see:

A la primavera el mar és fred.

Both are acceptable. The version with the comma simply marks the pause more clearly.

Could the word order be changed?

Yes. Catalan word order is fairly flexible.

These are both possible:

A la primavera, el mar és fred.
El mar és fred a la primavera.

The version with A la primavera first puts the time setting first, which often sounds very natural when introducing a general statement.

Why is there no word for it, like in English it is cold?

Catalan does not need a dummy subject like English it.

English says:

It is cold
It rains

But Catalan often simply uses the verb, or a real subject if there is one.

In this sentence, the subject is explicitly stated:

el mar

So Catalan does not need any extra pronoun equivalent to English it.

How is the sentence pronounced?

A simple pronunciation guide is:

ah luh pree-muh-VEH-ruh, el mar ess fret

A few helpful notes:

primavera has the stress on -ve-
és is pronounced like ess
fred often sounds like fret at the end, because final d is commonly devoiced

A more Catalan-like pronunciation, especially in Central Catalan, reduces some unstressed vowels, so the first part may sound closer to:

uh luh pree-muh-VEH-ruh

But the simple guide above is a good starting point.