They do enjoy it, but not for the reason one might think. While they vaguely have a concept of Halloween, they don’t actually fully understand it now. They know, at least, that humans honor(ed) their dead on this day, and the gastly line (especially haunter and gengar) seem to know that long ago, humans once worshipped them on this day. They also know that humans enjoy being scared right around this time and that if they do so, they’ll earn free candy.
But mostly, they know that Halloween is that day when humans hang up fake ghost-type decorations, and they find this incredibly entertaining.
The Pokédex According to Bill 