الحفلة اليوم.

Yes. In Modern Standard Arabic, this is a perfectly normal nominal sentence.

Arabic often makes present-tense sentences without a word for is / are / am. So:

• الحفلة اليوم
• literally: the party today
• natural meaning: The party is today

The idea of is is understood automatically in the present tense.

Because Arabic usually does not use a present-tense copula in simple sentences like this.

So English says:

• The party is today

But Arabic says:

• الحفلة اليوم

If you wanted the past or future, then Arabic would use other words, for example:

• كانت الحفلة أمس = The party was yesterday
• ستكون الحفلة غدًا = The party will be tomorrow

So the missing is is not an omission or slang here; it is standard Arabic grammar.

A common pronunciation is:

• al-ḥaflah al-yawm

If you read it with full formal case endings, it would be:

• al-ḥaflatu al-yawma

A few useful pronunciation notes:

• ح is a strong h sound made deeper in the throat than English h
• ة at the end of حفلة is usually pronounced like -a / -ah when stopping
• ال is pronounced clearly in both words here because ح and ي are moon letters, not sun letters

So you do not merge the l of ال into the next consonant here.

Because both are definite in meaning.

• الحفلة = the party
• اليوم = today

Even though اليوم literally looks like the day, in this kind of sentence it commonly means today.

So this is not strange Arabic. It is just the normal way to say today.

Compare:

• اليوم = today
• غدًا = tomorrow
• أمس = yesterday

The ة is called tāʾ marbūṭa. It often marks a noun as feminine.

So حفلة is a feminine noun. That is a grammatical feature, not necessarily something biological.

This matters because if you later describe it with an adjective, the adjective will usually be feminine too:

• الحفلة جميلة = The party is beautiful / nice
• حفلة كبيرة = a big party

When stopping after حفلة, the ة usually sounds like -a / -ah, not -t.

No. Here, اليوم is not an adjective.

It does not mean something like today’s party as an adjective phrase. Instead, it is functioning as the predicate or as a time expression: today.

That is why it does not have to agree with الحفلة in gender.

If it were an adjective, you would expect normal adjective behavior, like agreement in definiteness, gender, and number. For example:

• الحفلة الكبيرة = the big party

But اليوم is not an adjective like big or beautiful. It is telling you when the party is.

In fully vocalized MSA, you would usually analyze it as:

• الحفلةُ اليومَ

Here is the basic idea:

• الحفلةُ = the subject, so it takes ḍammah in full formal grammar
• اليومَ = a time expression (ẓarf), often understood as today, and it appears with fatḥah

In normal Arabic writing, these short vowels are usually not written, so you just see:

• الحفلة اليوم

That is completely normal.

Yes, you can, but the emphasis changes.

• الحفلة اليوم = The party is today
  This is the most straightforward way if you are talking about the party.

• اليوم الحفلة = Today is the party or It’s the party today
  This sounds more marked and puts extra focus on today.

So both can work, but word order in Arabic often affects topic and emphasis, even when the basic meaning stays close.

Yes, حفلة is a very common word and often means:

• party
• celebration
• concert
• event

Depending on context, Arabic may also use related words such as:

• حفل = ceremony, celebration, formal event
• حفلة موسيقية = concert
• حفلة عيد ميلاد = birthday party

So حفلة is a very natural word here.

It can look literal word-for-word as the party today, but in Arabic grammar this is normally understood as a full sentence:

• The party is today

Arabic often allows these compact two-word present-tense sentences. So even though English usually needs is, Arabic does not.

In the right context, the same words could also appear as a heading or note, where English might translate it more loosely as:

• The party today

But as a normal sentence, the standard understanding is:

• The party is today