De or het marathonsessie?

The correct Dutch article is:

de marathonsessie

Rule Words ending in -tie and -sie: almost always de. See the rule

How to use marathonsessie

the marathonsessiede marathonsessie
this marathonsessiedeze marathonsessie
that marathonsessiedie marathonsessie
our marathonsessieonze marathonsessie
your marathonsessiejouw marathonsessie
every marathonsessieelke marathonsessie
which marathonsessie?welke marathonsessie?
a big marathonsessie / the big marathonsessieeen grote marathonsessie, de grote marathonsessie