In the spectrum of wedding events, formality tends to build as the day goes along. While every wedding is different, morning and afternoon weddings tend to be more about the actual wedding ceremony and not so focused on the social function, while the evening wedding is often planned for maximum event impact.
On one hand, this makes it a bit easier to decide what to wear, in that one is, essentially, attending a soiree connected to a wedding (as opposed to a convention or a fund raiser, etc.), and need only pay attention to the conventions required for the formality level of the event. That said, there are certainly a lot of possible dress expectations … from “white tie” down to casual … and one would hope that the hosts would make this clear in the invitations. Of course, in these days of informality in communication, it is possible that this key info might not make its way into that envelope, so, when in doubt, it certainly doesn’t hurt to ask.
Of the options, the odds of a “white tie” wedding happening are fairly slim, unless one runs with Old Money or the Embassy set, and (unless it’s purely for scheduling purposes) its a rare bridal couple which opts for “casual” casual at an evening event … so one is left with a scale that pretty much runs from “black tie” to “business casual”. Again, knowing the intent of the hosts for what sort of party they’re having is key here.
If the invitation says “black tie” or “formal” or “black tie optional”, you’ll know they’re hoping for that “swell” look of a 1920’s bash with the guys in tuxedos and the gals in floor-length gowns. Obviously, if it’s “optional”, guys in dark suits won’t be out of place, and gals can probably go with a knee-length “little black dress”.
If the invitation says “semi-formal”, you’re pretty much looking at suits and ties for the guys, and anything that would fit in a dressy dinner setting for the gals. Sometimes the specification will just be for “dressy”, and this is probably looking for this level of attire as well.
If the invitation says “casual”, your best bet, especially for an evening event, is assuming this means “business casual” and not “going to the ballpark” casual. Guys in slacks and sport coats (possibly without ties), and gals in outfits and combinations suitable for office wear (although avoiding the “presentation suit” end of the closet).
As always, unless you know for an absolute fact that the wedding couple (and their families) are dead-set on living their lives in shorts, t-shirts, and sandals … and are planning the event to reflect that … anything less than “business casual” is taking a huge risk. You don’t want to be the one there who “knows the bride”, but didn’t expect that her family had convinced her to “play dress-up” this one day!
Also, those wedding-specific rules come into play … nobody should be wearing white other than the bride, you should avoid color ranges too close to the maids of honor dresses, and if the event is not specified as formal avoid the tuxedo. And always give a “bride’s grandmother” thought to how your selected outfit is likely to come across.