Ostern ist morgen.

Because Ostern is the name of a holiday, and in German holiday names are usually used without an article.

• Ostern ist morgen. = Easter is tomorrow.
• Weihnachten ist bald. = Christmas is soon.

In some other contexts, holiday words can appear in fixed expressions like zu Ostern or an Ostern, but in a simple sentence like this, no article is needed.

• Ostern = the subject
• ist = the verb (is), from sein
• morgen = an adverb of time meaning tomorrow

So the structure is:

Subject + verb + time expression

That is very normal in German.

Because Ostern is treated as singular.

Even though Ostern ends in -n, it is not plural here. It is the singular name of the holiday Easter, so it takes the singular verb:

• Ostern ist morgen.

Not:

• Ostern sind morgen.

Because here morgen means tomorrow, and in that use it is an adverb, so it is written with a lowercase m.

Compare:

• morgen = tomorrow / adverb
• der Morgen = the morning / noun

Examples:

• Ich komme morgen. = I’m coming tomorrow.
• Der Morgen ist kalt. = The morning is cold.

So in Ostern ist morgen, lowercase morgen is correct.

Yes, but only when it is the noun Morgen, which is capitalized.

Compare:

• morgen = tomorrow
• Morgen = morning

In your sentence, it clearly means tomorrow because:

1. it is lowercase, and
2. it fits grammatically as a time adverb

So:

• Ostern ist morgen. = Easter is tomorrow.

Not:

• Easter is morning.

Because time expressions often come after the verb in simple German sentences.

German main clauses usually put the conjugated verb in second position, and other elements often come later:

• Ostern = position 1
• ist = position 2
• morgen = later in the sentence

So Ostern ist morgen is completely natural.

Yes. That is also correct.

Both mean the same thing, but the emphasis is slightly different:

• Ostern ist morgen. → focuses first on Easter
• Morgen ist Ostern. → focuses first on tomorrow

This is a common feature of German word order: the element in the first position often gets a little more emphasis or sets the topic.

Yes. Ostern is still a noun, specifically a proper name for a holiday.

In German, all nouns are capitalized, including proper names and holiday names. That is why Ostern has a capital O, even without an article.

Only indirectly.

• Ostern is in the nominative case because it is the subject.
• morgen is an adverb, so it does not take a case here.

There is no visible article like das or der to show the nominative, but grammatically Ostern is still nominative because it is the thing that is something.

A simple English-friendly approximation is:

OH-stern

A bit more accurately:

• O as in a long o
• stern sounds somewhat like shtairn in many standard descriptions, but without exaggerating it

The st at the beginning of the second syllable is pronounced sht in standard German:

• Ostern ≈ OH-shtairn

If you are just starting, OH-stern is a decent first approximation.