Precisely because the urban standard for cocktail attire is potentially at odds with the locale, contacting the host is vital. Going with the conventional interpretation of "cocktail attire" is a dangerous game in this case.
The implication that "bumpkinland" was implied is misconstrued. I have been to cocktail receptions in Napa Valley, and a nice sport coat is what the gentry there wear to such an event, it was my observation.
Here is a cocktail event at the French Laundry, arguably one of the premier restaurants in that area. Naturally, I have no idea what, if anything, the invitation said, but this is what real people wear there:
Shot at 2010-05-31
I agree that it would be wise to contact the host, but not at all because there is some sort of varying standard for cocktail attire. It is a definite dress code, whether one is in the city, suburbs, countryside, or wherever. Dress codes do not fundamentally change depending on the locale. For instance, "black tie" would
not mean a leather vest and black bolo if one were in the southwest.
Of course events requiring various dress codes may be more or less common, depending upon the location. One might presume, for instance, that it would be far more likely to be invited to several black tie events a year if one lived in Manhattan, than if one lived in Bozeman.
That does not mean that black tie (or cocktail attire) events require
different attire in Bozeman than in Manhattan. It simply means that that particular attire is likely to be called for less frequently, and more often a more casual attire specified (or presumed.)
The photo you posted shows a group of people who (almost to a person) are wearing casual attire. The use of this photo to attempt to define cocktail attire is, unfortunately, terribly misleading.
To begin with, you yourself state that you have "no idea" what, if any' attire was specified on the invitation. That alone would render the photo useless as an example.
Even if we were to have a copy of an invitation to this event, and it clearly stated "cocktail attire," the only conclusion we could draw would be that the majority of the people in the photo are inappropriately dressed.
And of course merely because an event is a cocktail party, one can not presume that cocktail attire will be specified ...nor does it mean that
whatever happens to be worn at it is automatically "cocktail attire." (By the way, I do not believe I see any cocktails in that photo, though there are a few glasses of wine visible.)
Were I to hold a daytime, outdoor event in the Napa Valley, I would very likely specify casual attire, especially if it were on or adjacent to a lawn. Perhaps if it were a late afternoon event, held in the courtyard of a winery or manor, cocktail attire would be appropriate.
I would also caution anyone to not base their attire for a wedding in the Napa Valley upon what the local population generally wears. Unless all in attendance are local, very few will be likely to know or care about local dress norms... and ...
it's a wedding, which (unfortunately) is one of the few remaining occasions that many people ever bother to dress for.
The event the OP was invited to is a night time reception, which is part of a three day wedding, and for which cocktail attire was specified. He asked exactly what that means. I, Andy, and others have given the correct and exact meaning. You serve only to mislead him with your presumptions which are based upon inexact notions.
You are correct, as I mentioned previously, that it would be valuable to contact the host. The reason for doing so would not be because cocktail attire has various meanings, but to confirm that the host is indeed using the term in its correct definition.