Where does the idea come from that anything in someone else's house is fair game? I like to display cards (what else would you do with them?) but I don't see them as public property. Of course people can see the fronts, so saying 'what a lovely card' or similar is fine, but reading the messages inside is a step or three too far for me.
Wall calendars are for others in the house to know what people are doing, but not for visitors to look at appointments, and commenting on them is outrageous, IMO. It just wouldn't occur to me to do it. I mean, who needs to know that their neighbour is getting their haemorrhoids sorted out on Tuesday?
We're all different, I suppose.