If you receive a letterpress or embossed envelope in the mail, chances are a skirt or suit is required. A Facebook message, unless it specifies otherwise, likely calls for a much more casual look.
The venue imparts plenty of info. A wedding at a country club has a much different vibe than one on a beach or in a warehouse. If the event is at an unfamiliar spot, try searching online for photos ahead of time.
Time of Day
The start time of a party sets the tone for the celebration. Typically, the earlier an event, the less formal it is. Remember to take into account seasonal changes: Sunset comes sooner in the fall than in the summer.
Do what you can to suss out the cadence of the event. A limited time frame—say, if a shower goes from 3 to 5 p.m.—calls for different attire than a wedding that begins with an early afternoon ceremony and continues on to late-night dancing.
When in doubt, ask for help. Check with a friend who is going to the same event or, better yet, call a few to get a wider sampling. If it's a wedding, though, try not to bother the bride—she’s busy enough. Reach out only as a last resort.