De or het reisplanning?

The correct Dutch article is:

de reisplanning

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

How to use reisplanning

the reisplanningde reisplanning
this reisplanningdeze reisplanning
that reisplanningdie reisplanning
our reisplanningonze reisplanning
your reisplanningjouw reisplanning
every reisplanningelke reisplanning
which reisplanning?welke reisplanning?
a big reisplanning / the big reisplanningeen grote reisplanning, de grote reisplanning