So, I had a thought. It would be really nice to write a reminder to myself, and have it show up on a scheduled day to remind me of something. For example, I could mail myself a note to that would remind me to pick up an upgraded tool, check my kegs, visit the quarry, give someone a birthday present and so on. These notes would show up in the mailbox after waking up on the scheduled day, just like notes from NPCs etc. Several hurdles I see: Need to be able to write the message in-game, so would need some sort of UI for that Need to be able to schedule the message delivery - again, some sort of UI, though the calendar is already there and might be useful. I tried poking around with Storm, but it's wayyy over my head re: programming mods; if anyone else thinks this could be useful and would be able to implement it, you'd have my eternal gratitude!
Second this. Alternate/additional idea: give yourself a quest in the journal as the note and have it trigger the exclamation button in the UI on the marked day.