The one more day thing has come up from so many separate sources, I really do wonder what it means.
I really do think there was a note, just that it was probably taken out of context by Heather that alluded to an event on Saturday (a discussion? family dinner event to which she was invited?). Art had a tone that could be melodramatic at times. While I think he knew he wasn't long for this Earth, he planned on being around another year or more at least.
The Sheriff/Coroner may taken this just in case and so she did in fact "read the note" before handing it to them. If it were a email address - these were reassigned so she didn't have a copy by the time she started telling folks about this. In context with the families reports ,they may have dismissed it. There may have been more than one with similar content from the days before lamenting his old age ailments, who knows.
Until anything else surfaces this is my absolute best theory on all of it. Probably not exactly correct, but close.