Learn Russian: Сегодня христиане празднуют Рождество. - Today Christians are celebrating Christmas.

The singular form is христианин. Nouns for people (like nationalities or religious groups) that end in -анин or -янин follow a special plural rule. They drop the -ин and add -е. So, христианин drops -ин to become христиан-, and adding -е gives us христиане.

Does the stress change between the singular and plural forms of христианин?

Yes! In the singular (христиани́н), the stress is on the final syllable. In the plural (христиа́не), the stress shifts back one syllable to the -а-. This backward stress shift is very common for nouns ending in -анин / -янин when they become plural.

How does the verb празднуют come from the infinitive праздновать?

Verbs ending in -овать or -евать have a special conjugation pattern in the present tense: the -ова- or -ева- drops out and is replaced by -у- or -ю-. That is why праздн-ова-ть turns into праздн-у-ют for the 'they' (third-person plural) form.

Why doesn't the ending of Рождество (Christmas) change, since it is the direct object being celebrated?