I probably usually cheat on ‘X day’; outside of writing about xorn and Xavier (Professor X) and xylophones, fitting this ‘X day’ is a challenge on its own.
Sometimes an NPC grows beyond the intended use. An NPC intended for a single encounter can end up being used in multiple encounters or even multiple adventures because the PCs keep returning. For instance, the wizard Romney in our Saturday night game was expected to be important for one adventure, but we hung around long enough that we ended up going to him several times (and I even spent a winter with him as an ostensible ‘apprentice’, trading an upgrade to his lifestyle for new spell knowledge).
Sometimes, it’s even more accidental.
Long ago, the PCs were in the Northlands and passing through a village when they decided they wanted to find some potions. I made up a throwaway NPC (Jrom — pronounced much like ‘Jerome’, but without that first ‘e’).
In entity template terms I might have had something like
Name Jrom
Theme Half-deaf hedge witch
Reward Minor potions and spell casting, stories of things only he remembers… if you can get through to him.
Appearance Withered old man with ‘Bernie hair’ (I didn’t know of Bernie Sanders then, but looking back the description fits) and an unusually complete set of teeth (benefits of good living?). Often has his head cocked to try to hear better.
I honestly hadn’t thought much about him. PCs got to talk to him, I got to mess with them (“we’re looking for potions of healing” “Long way from the ocean here, boy, and there ain’t no seals no how”), the game moved on.
Several months later the group had returned from the north and been sent far, far to the west, to Trollskov, a somewhat Russia-inspired place. They were trying to track down a murderer that had been hunting down city folk and tearing out their hearts, and were talking with the governor and his advisors. One of them mentioned that this had happened before, long ago.
I had expected the party to go to the ‘House of Lore’ (I forget what pseudo-Russian name I gave the group) when one of the PCs said, “too bad we’re not up north, Jrom probably knows this”.
Never one to waste an opportunity…
“I am Aleksander Jromovich… why would you want to talk to my father? He’s a half-deaf old man.”
You can imagine how fast they beat feet to the old man’s house, where they met a withered old man with ‘Bernie hair’ and foul breath (from the decades of vodka abuse), and a surprising amount of information about the Bad Times so many decades ago… once they got through to him what they were looking for.
At that point I knew I had an NPC that would be, if they could find him, just about anywhere they needed and able to provide thought-lost information… if they could get through to him.