Latin-derived nouns ending in -um change to -a in the plural form.
Millennium is regular. It follows the Latin -a rule.
Use plural verbs: "The millennia are ready" (not "is ready").
Yes. The plural millennia is standard across all English variants.
The word millennium follows the standard English pluralization rule. When forming the plural, millennium becomes millennia. This follows the Latin -a pattern, which is one of the most common plural rules in English.
Understanding why millennium becomes millennia helps with spelling and pronunciation. Latin-derived nouns ending in -um change to -a in the plural form.
When using millennia in writing, remember that it functions as a plural noun in sentences. It pairs with plural verbs and pronouns. For example: "The millennia are ready" (not "is ready"). This subject-verb agreement is essential for grammatical correctness.