De or het haarwassing?

The correct Dutch article is:

de haarwassing

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

How to use haarwassing

the haarwassingde haarwassing
this haarwassingdeze haarwassing
that haarwassingdie haarwassing
our haarwassingonze haarwassing
your haarwassingjouw haarwassing
every haarwassingelke haarwassing
which haarwassing?welke haarwassing?
a big haarwassing / the big haarwassingeen grote haarwassing, de grote haarwassing