Proslava je u subotu.

• Proslava = celebration, party, or festivity depending on context. It is the subject of the sentence.
• je = is, the 3rd person singular present tense of biti (to be).
• u = a preposition. Here it is used in a time expression meaning on.
• subotu = Saturday in a changed form required by the preposition u in this expression.

So structurally, it is:

Proslava + is + on Saturday

Because Serbian changes nouns depending on their role in the sentence. This is called case.

The basic dictionary form is subota (Saturday). After u in this time expression, Serbian uses the accusative, so subota becomes subotu.

For many feminine nouns ending in -a, the accusative singular changes -a to -u:

• subota → subotu
• škola → školu
• proslava → proslavu

So u subotu means on Saturday.

This is just a difference between the two languages.

In English, we say:

• on Saturday

In Serbian, the natural expression is:

• u subotu

Even though u often means in or into, in expressions with days of the week it usually corresponds to English on.

So you should learn u + day of the week as the normal Serbian pattern:

• u ponedeljak = on Monday
• u utorak = on Tuesday
• u sredu = on Wednesday
• u subotu = on Saturday

Je is the present-tense form of biti (to be) for he/she/it is.

In this sentence, Serbian uses je just like English uses is:

• Proslava je u subotu. = The celebration is on Saturday.

This is a very common form, and you will see it all the time:

• On je student. = He is a student.
• Ona je kod kuće. = She is at home.
• Sastanak je sutra. = The meeting is tomorrow.

Because Serbian does not have articles like English a/an and the.

So proslava can mean:

• a celebration
• the celebration

The exact meaning depends on context.

In this sentence, English usually translates it as The celebration is on Saturday, but Serbian simply says Proslava je u subotu without any article.

Yes. Serbian word order is more flexible than English word order.

The neutral version is:

• Proslava je u subotu.

But you could also say:

• U subotu je proslava.

Both mean essentially the same thing, but the emphasis changes a little.

• Proslava je u subotu. → neutral, starting with the event
• U subotu je proslava. → puts more focus on when it is

English is less flexible here, but Serbian often moves elements around for emphasis or style.

It can mean several similar things depending on context, such as:

• celebration
• party
• festivity
• sometimes even ceremony

For example:

• a birthday party can be described as a proslava
• a holiday celebration can also be a proslava

So the most natural English translation depends on the situation.

Because in Serbian, names of days of the week are normally not capitalized.

So Serbian writes:

• ponedeljak
• utorak
• subota

not:

• Ponedeljak
• Utorak
• Subota

This is different from English, where Monday, Tuesday, and Saturday are capitalized.

Yes, it is completely natural and fully grammatical.

Proslava je u subotu. is a normal everyday sentence. It sounds neutral and standard.

A Serbian speaker would easily understand it as something like:

• The celebration is on Saturday.
• The party is on Saturday.

So this is a very useful pattern to learn:

• [event] + je + u + day of week

For example:

• Sastanak je u petak. = The meeting is on Friday.
• Koncert je u nedelju. = The concert is on Sunday.

In standard Serbian, you normally keep je in a sentence like this.

So:

• Proslava je u subotu. = correct

Leaving it out would usually sound incomplete or nonstandard in this context.

Serbian sometimes omits forms of to be in certain styles, headlines, or informal notes, but for a normal full sentence, je should stay.

By itself, u subotu usually means on Saturday, and the exact interpretation depends on context.

It can refer to:

• the coming Saturday
• a specific Saturday already mentioned
• sometimes a habitual situation, depending on the sentence

For example:

• Proslava je u subotu. = The celebration is on Saturday.
  Usually this means one specific Saturday.

But in a habitual context:

• Idem u grad u subotu. = I go into town on Saturday / on Saturdays.
  Context tells you whether it is one Saturday or a repeated action.

So the phrase itself is the same; context decides the exact meaning.