Сегодня Рождество.

In Russian, the verb быть is usually omitted in the present tense when you are simply saying that one thing is another.

So:

Сегодня Рождество. = Today is Christmas.

Literally, it looks like Today Christmas, but that is normal Russian.

Compare:

• Вчера было Рождество. = Yesterday was Christmas.
• Завтра будет Рождество. = Tomorrow will be Christmas.

In the past and future, Russian does use forms of быть.

Сегодня is an adverb meaning today.

It does not change for case, gender, or number. It simply gives the time of the situation.

So in this sentence, сегодня tells you when: today.

The stress is on the middle syllable: сего́дня.

A rough English-friendly pronunciation is si-VOD-nya.

One important detail: in this word, the letter г is pronounced like v, not like a normal hard g. That is why learners are often surprised by how сегодня sounds.

Here Рождество is in the nominative singular.

It is the basic dictionary form of the noun. In a simple present-tense sentence like this, Russian uses the noun directly without any article and without an expressed is.

So you should think of Рождество here as the plain noun naming the holiday.

Рождество is a neuter noun.

You can tell from the ending -о, which is a very common neuter ending in Russian.

So if you later use adjectives or past-tense verbs with it, they will agree in neuter:

• Русское Рождество
• Рождество было тихим

Russian does not have articles like a or the.

So Рождество can mean Christmas without adding anything.

Russian relies on context, word order, and meaning instead of articles.

Because it is the name of a holiday.

In Russian, holiday names are normally capitalized, so Рождество is written with a capital letter when it means the holiday Christmas.

Sometimes, yes, but Сегодня Рождество is the most neutral order here.

Russian word order is flexible, but different orders can sound more marked or emphatic. For a simple statement, Сегодня Рождество is natural and straightforward.

If you move the words around, the sentence may sound poetic, contrastive, or context-dependent rather than neutral.

Normally, no.

Using есть here sounds unnatural. In modern Russian, есть is not used as a normal present-tense is in sentences like this.

So the natural version is:

Сегодня Рождество.

not

Сегодня есть Рождество.

That depends on the cultural and religious context.

In Russia, Рождество often refers to Orthodox Christmas, which is celebrated on January 7 by the Russian Orthodox Church.

But in other Russian-speaking contexts, or when speaking about Western traditions, it can also refer to December 25.

So the word itself means Christmas, but the exact date depends on who is speaking and what tradition they mean.