De or het peilopening?

The correct Dutch article is:

de peilopening

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

How to use peilopening

the peilopeningde peilopening
this peilopeningdeze peilopening
that peilopeningdie peilopening
our peilopeningonze peilopening
your peilopeningjouw peilopening
every peilopeningelke peilopening
which peilopening?welke peilopening?
a big peilopening / the big peilopeningeen grote peilopening, de grote peilopening