De or het millenniumviering?

The correct Dutch article is:

de millenniumviering

Rule Words ending in -ing: almost always de. See the rule

How to use millenniumviering

the millenniumvieringde millenniumviering
this millenniumvieringdeze millenniumviering
that millenniumvieringdie millenniumviering
our millenniumvieringonze millenniumviering
your millenniumvieringjouw millenniumviering
every millenniumvieringelke millenniumviering
which millenniumviering?welke millenniumviering?
a big millenniumviering / the big millenniumvieringeen grote millenniumviering, de grote millenniumviering