No, no, I wouldn't want to intrude on your date. Besides, I'm having my own date that night.

[ Viktor gestures Adachi to follow and he turns to head to one of the men's outlets nearby, handing back his letter. ]

To be honest, semi-formal is pretty fluid in its meanings, but it's basically less formal than a tuxedo but more formal than jeans or khakis. There's a lot between those two extremes though which is why it depends on the setting of the event you're going to. A suit and tie is usually the best bet though.