Sometimes when I design scenarios, I realize that my mental image of the adventure is only exciting because it takes place at night. When the players actually play through the adventure, they might foil my plans by simply deciding to investigate the haunted forest during daylight hours.
In horror movies, they always find a reason for the scary parts to take place at night (even if that reason is just sheer stupidity). In a tabletop RPG where the PCs have free will, things aren't quite as easy.
When an adventure needs to be set at nighttime in order to be dramatic, how can I make sure that it actually takes place at night?
Best Answer
I hate railroading and GMs breaking immersion to enforce their hackneyed vision of "the plot" so I figured I needed to contribute another perspective in the answers.
Try not being completely in love with the specific idea of "there at night" you have. So they can go there during the day, and it's still scary. You can employ fog, rain/snow, and/or thick underbrush - it's just as gloomy and vision-reducing as night. (similar to @Ravn's answer) Or a very heavy hand of "the sun moves supernaturally fast and disappears over the horizon!"
The local authorities don't want people going there, and sneaking there under cover of night is the only way to evade detection. (A common justification in horror stories).
Are they going there to specifically confront/see a ghost or interact with the supernatural? Well, "They mostly come out at night." (similar to @evilcandybag's answer)
Have the forest be large enough you can't get through it in a day - thus by necessity, they'll be in it at night. (or, @OpaZitiZen's suggestion of inserted delays) You can trap them there too, by a party member or NPC or pack animal being sick or wounded, physical hazards blocking progress or retreat, or other more tangible dangers (damn, an orc warband! Well, spooky forest is less directly fatal, let's go to ground here...)
Like most event driven adventures, have some kind of time pressure on where they have to go there tonight or else lose the chance/someone else will die/etc. (Also a common horror story justification)
Why does Buffy patrol the graveyard at night? 1. It's when the vampires come out and 2. It's less obvious to the locals that doing it during the day. Why is Ash in the spooky forest at night? In Evil Dead, Ash is out there at night because 1. he gets KO'd most of the day, 2. the bridge is out, and 3. the sun sets with unsettling rapidity. Not all horror stories have people in the scary place "just because," well written ones do have justifications for it - actually read some of them and take the ideas.