At a black-tie event, the men dress in tuxedos or dark suits and the women wear fancy dresses.

VOA: special.2009.12.28

but also I think they're excused to dress up in fancy clothes

但是，我觉得他们是要借此机会盛装打扮一下，

英国皇家赛马会 - SpeakingMax英语口语达人