For what it's worth I had the same problem as the OP. I looked at the calendar, thought "well, I don't see days, but most calendars I've seen start on Sunday, and went through the whole puzzle on that assumption. Took me forever to finally go back and reconsider that assumption and swap my last two names to finally make the rest of the remaining clues make sense. And then when I saw the hint from Merulon about the hint in the agenda, I went back and looked and was even more convinced I wouldn't have gotten it if I'd seen that. A New Year's Eve party on Sunday might have clued me in if I'd noticed that, but a New Year's Party? Is that a NYE party? Or a party being thrown on New Year's Day? Obviously, with the benefit of hindsight, it's the former in the context of this puzzle, but I wouldn't have assumed so.
However, that problem aside, quite enjoyed it.