Si la data canvia, escrigui-la al calendari perquè ningú no l'oblidi.

Yes. In Catalan, data very commonly means a calendar date.

So in this sentence:

• la data = the date

It can be a false friend for English speakers, because English data usually means information, but Catalan data often refers to a day on the calendar.

Because after si in a real, possible condition, Catalan normally uses the present indicative, not the future.

So:

• Si la data canvia = If the date changes

This is the same basic idea as in English:

• If it changes, ...
• not usually If it will change, ...

Then the main clause can use a command:

• Si la data canvia, escrigui-la...

So the pattern is:

• si + present indicative
• then a main clause such as an imperative, present, or future

Here escrigui is a formal singular command addressed to vostè.

It comes from the present subjunctive, but in this context it functions as the imperative for formal you:

• escrigui = write / please write (to one person, formally)

Compare:

• informal singular: escriu
• formal singular: escrigui
• informal plural: escriviu
• formal plural: escriguin

So this sentence is speaking politely or formally to one person.

Because la is the direct object pronoun referring back to la data, and in Catalan, object pronouns are attached to the end of an affirmative imperative.

So:

• escrigui-la = write it
• la = it, referring to the date

This attached pronoun is very normal after:

• affirmative commands
• infinitives
• gerunds

That is why you get:

• Escrigui-la = Write it

but in other contexts the pronoun may go before the verb.

Catalan normally uses a hyphen when a pronoun is attached to an affirmative imperative.

So:

• escrigui-la
• digui-ho
• miri'l

The hyphen makes the verb + pronoun combination clear in writing.

Because al is the contraction of:

• a + el = al

So:

• al calendari = to/on the calendar

This is very common in Catalan:

• al llibre = a + el llibre
• al carrer = a + el carrer

Here al calendari means writing the date in/on the calendar.

Here perquè means so that, introducing a purpose clause.

So:

• perquè ningú no l'oblidi = so that nobody forgets it

The accent matters:

• perquè = because / so that
• per què = why / for what reason

In this sentence, it means so that, not because.

A useful clue is the verb that follows:

• oblidi is subjunctive, which fits a purpose clause very well.

It is a kind of negative reinforcement, and it is normal in Catalan.

• ningú = nobody / nobody
• no = negative particle

So:

• ningú no l'oblidi literally looks like nobody not forget it, but that is standard Catalan structure, not an error.

For English speakers, it feels like a double negative, but Catalan handles negation differently.

Also, when a negative word like ningú comes before the verb, the no is often possible and quite common, especially in careful or written language.

So ningú no l'oblidi is perfectly natural.

Because after perquè meaning so that, Catalan often uses the subjunctive to express purpose, intention, or desired result.

So:

• perquè ningú no l'oblidi
• literally: so that nobody may forget it
• natural English: so that nobody forgets it

That is why you see:

• oblidi = present subjunctive

not:

• oblida = present indicative

This is a very common pattern in Catalan.

Because pronoun placement depends on the verb form.

In this sentence:

• escrigui-la: affirmative command → pronoun attached after the verb
• l'oblidi: normal finite verb in a subordinate clause → pronoun before the verb

So Catalan changes pronoun position depending on the structure.

Also, l' is just the shortened form of la before a vowel:

• la oblidi becomes l'oblidi

And it still refers to la data.

Yes. Both pronouns refer to la data.

• escrigui-la = write it
• ningú no l'oblidi = nobody forgets it

Catalan avoids repeating la data by using object pronouns instead.

That makes the sentence more natural and less repetitive.

If you were speaking to one person informally, you would usually say:

• Si la data canvia, escriu-la al calendari perquè ningú no l'oblidi.

The main change is:

• formal escrigui-la
• informal escriu-la

The rest of the sentence can stay the same.