Hopefully it won't be a day every 100 years though. ^^
And what would happen to you if you were in the area when the window closes? Would you "fly" out the nearest exit, or would you just get stuck there until the map is available again (unless you can teleport), or would you die?
if you got stuck there after it vanished you could stay indefinitely, and could leave just as if it were still connected to the main world, but if you ever left you couldn't ever go back.
Soo the signpost on the outside vanishes or crumbles or something, while the signpost on the inside is still there, and maybe when you try to leave pet rock informs you that you can't return until the area is available again and asks you if you're sure?
A pickle factory. Maybe it can be done in the realistic grungy photograph style with giant swinging doors that open and close to allow people in. And you can get out by jumping over the wall. Fascinating.
It's a useless piece of info which I can't flush from my brain. I got to go inside once, but didn't realize that I should spend every single turn in there that day. It won't be back for years from how it sounds.
Special events are another fun way to do this. Like holidays for the Giants - surely each of them has a special day? Donations would count for extra on their holidays, and maybe they would have something special for their favors.