Yeah, I kinda figured that might have been the tragedy. But the question is, did they forget (and attribute that to the fact that it was so soon after) or did they just assume that you weren't going to have the party after all?
Either way, it's rude. If they forgot, well, ok, yes, that was a pretty horrible time, but life did go on. People still had doctor's appointments and work meetings, etc. And social engagements. Calendars didn't just get ignored -- assuming none of these people were personally affected by the attacks*. And if they assumed, well, they should have asked, huh?
*To clarify, I think that the entire country was personally affected by the attacks. What I mean is, if you were there or knew somone who was there.