I don't know when the phrase "fashionably late" became popular and regularly practiced, but it has taken off with abandon. In fact, if you're prone to promptness like me, chances are you'll be the only person at any given party for at least a half-hour after the indicated start time. That's because arriving extremely late has become the standard. In stark contrast, most etiquette experts agree that the window of fashionable tardiness is a mere five to 15 minutes [sources: Martin, Emily Post]. The train seems to have left the station on this etiquette infraction, with no signs of turning back. Since it makes my eye twitch to be late, I'll enjoy the small crowd and extra finger food while everyone else trickles in slowly.
The only time a late arrival may be acceptable is when it's a cultural norm among your family, friends or social group. If that's the case, then an on-time arrival might mean catching the hostess running frantically in her underwear to take something out of the oven.