Vi ska fira hennes födelsedag på lördag, så glöm inte att säga grattis när du kommer.

Ska fira expresses a plan or intention: we’re going to celebrate / we will celebrate.

So:

• Vi ska fira hennes födelsedag på lördag = We’re going to celebrate her birthday on Saturday

Swedish can also often use the present tense for future meaning when the time is clear:

• Vi firar hennes födelsedag på lördag

That is also natural. Using ska just makes the planned-future meaning a little more explicit.

Because ska is a modal verb, and modal verbs in Swedish are followed by the infinitive without att.

Examples:

• ska fira
• kan komma
• vill äta
• måste gå

But glömma is not a modal verb, so when it is followed by another verb in the infinitive, Swedish normally uses att:

• glöm inte att säga
• hon försöker att förstå (though att can sometimes be omitted with some verbs)

So in this sentence:

• ska fira = no att
• glöm inte att säga = att is needed

Because sin/sitt/sina are reflexive possessive words. They are only used when the owner is the same as the subject of the clause.

Here, the subject is vi (we), not hon (she):

• Vi ska fira hennes födelsedag = We are going to celebrate her birthday

Since her refers to someone other than the subject vi, Swedish uses hennes, not sin.

Compare:

• Hon firar sin födelsedag = She is celebrating her own birthday
• Vi firar hennes födelsedag = We are celebrating her birthday

After a possessive like min, din, hans, hennes, vår, etc., Swedish normally uses the noun in its basic singular/plural form, not with a separate article and not with the definite ending.

So Swedish says:

• min bok = my book
• hennes födelsedag = her birthday

Not:

• min boken
• hennes födelsedagen

The possessive already makes the noun definite enough, so Swedish does not add another definite marker there.

På lördag means on Saturday, usually the coming Saturday.

Swedish often uses på with days in expressions like this:

• på måndag = on Monday
• på fredag = on Friday
• på lördag = on Saturday

This is just the normal preposition Swedish uses here, even though English uses on.

A few useful comparisons:

• på lördag = this coming/on Saturday
• i lördags = last Saturday
• om lördag is not used in this meaning

Here så means so and connects the two parts of the sentence:

• Vi ska fira hennes födelsedag på lördag, så glöm inte att säga grattis...

In other words:

• We’re celebrating her birthday on Saturday, so don’t forget to say congratulations...

It shows a result or consequence: because it is her birthday celebration, you should remember to say grattis.

Yes. Grattis is the normal Swedish word for congratulations, and it is also what people commonly say on birthdays.

So on someone’s birthday, you can simply say:

• Grattis!

You can also say:

• Grattis på födelsedagen! = Happy birthday!

In everyday Swedish, just Grattis! is very common and natural.

Yes, absolutely. In real life, you would often simply say:

• Grattis!

But in this sentence, säga grattis means to say congratulations.

So:

• glöm inte att säga grattis = don’t forget to say congratulations

This is a very natural way to talk about the action of congratulating someone.

You may also see:

• gratulera henne = congratulate her

But säga grattis is often more everyday and conversational.

Because när du kommer is a subordinate clause introduced by när (when). In Swedish subordinate clauses, the normal word order is subject + verb:

• när du kommer

Not:

• när kommer du

The inverted order (verb + subject) is used in main questions:

• När kommer du? = When are you coming?

So:

• ...när du kommer = ...when you come
• När kommer du? = When are you coming?

This is very common in both Swedish and English. After time words like när (when), Swedish often uses the present tense to talk about the future.

So:

• när du kommer literally looks like when you come/are coming
• but it means when you come / when you arrive in the future

Compare English:

• Call me when you arrive
• not usually when you will arrive

Swedish works similarly here.

In this context, it can be understood as come or arrive. Both fit naturally in English.

• när du kommer = when you come
• very naturally: when you arrive

Swedish komma often covers both ideas, depending on context. Since this is about showing up at a birthday celebration, when you arrive is probably the most natural interpretation.