Because I'd somehow misread an invitation that explicitly said "Friday, the 28th" and put it on my calendar for "Saturday, the 29th." I looked back and double checked the invitation at least twice -- and the misreading persisted. I might have gotten it right if I'd explicitly asked for confirmation, but it's actually been a while since the last such incident I can remember.
Still, this sort of thing has plagued me for as long as I've maintained my own appointment book (so, most of my life), and I've run out of new ways to deal. I do now keep my calendar in the cloud, synced through MacBook and iPhone and web browsers, and alarms on my iPhone are my friend, but that only works if I've put the right data there in the first place.
If you send me an invitation, and there's any way to send an .ics or similar file -- which has all the date and time info inside it, and which my calendar can import and deal with without my intervention -- I hope you will. Most of the online invitation managers (Evite, Anyvite, Facebook, and the like) allow me to export such files (though they each have bugs, like all Evite events seem to be 1 hour long), and the worst possible errors (as with last night) are avoided.
Lacking that, invitation by voice is more likely to be heard right, and if I'm in a place where I can put it right into my calendar -- all the better.
Hopefully the events on my calendar for the next few days are correctly set.