Don’t be afraid to ask questions. If you’re ever unsure about what constitutes appropriate attire for an event, ask the person hosting it. They are, after all, the most invested in avoiding confusion and uncomfortable situations.
Do go for something tailored. A tuxedo is too much, but a suit or blazer/trousers combo is required.
Don’t upstage the guest of honour. If you’re attending a birthday or any other occasion in which the spotlight is meant to be on someone specific, leave the bright colours and flashy accessories at home.
At last, choosing cocktail dress should dependent on the occasion, if you know the dress code isn’t super strict, explore colours (always dark, in this case) such as burgundy, olive green or khaki. Patterns should be kept minimal, sticking to windowpane check (small scale) and pinstripes (never chunky or thick).