Just got an STD for a wedding next summer and I went to the website. Under attire it mentions that it's "Semi-Formal/Formal". The reception is at a country club... but it might be starting around 3pm if they don't have a gap (though they don't actually specify reception start time).
My guess is they are hoping for cocktail attire but don't know how to word it (not that it should really be worded at all).
I'm trying to think of a nice way to ask...