Why is it das Millennium?

Millennium ends in -ium, and nouns ending in -ium are neuter: in a German dictionary, 100% of them take das.

All nouns ending in -ium
Sense 1
das

Meaning

[1] Time period of thousand years

[1] Zeitabschnitt von tausend Jahren

Examples

[1] It took several Millennia

[1] Es dauerte mehrere Millennien.

Declension

1 Singular Plural
Nominativ das Millennium die Millennien
Genitiv des Millenniums der Millennien
Dativ dem Millennium den Millennien
Akkusativ das Millennium die Millennien

Dictionary entries include material from Wiktionary. Wiktionary