Here's somthing frustrating

My boss invited me to an event, for which I had to go buy a cocktail dress as I didn't own one. But apparently, we aren't going to the event, but he didn't think that it was important to tell me, so I've been going along as though I need to plan for a big fancy political event and will need to prepare to leave another event early (the latter event requires a great deal of my attention, so I have to make sure no crises will occur, which has already been a great expenditure of effort). At what point did he think it was ok not to tell me this, even though he's been emailing me all day about other stuff? He told the other invitee, my co-worker, and she told me, but why was it that he bothered to tell her and not me?

UPDATE: It turns out that the invitation may have been rescinded (from both myself and my co-worker) because a mistress was invited instead and the number of available slots was miscalculated. Pure hearsay at this point, but oooh it's juicy!