I get a lot of queries from North American brides and grooms about what dress is appropriate for formal weddings that straddle the dividing line between day and evening. I answer this question in the Guide’s Formal Evening Weddings page but it’s worth repeating here.
To recap the standard etiquette, if the ceremony and reception take prior to 6:00 PM (give or take an hour) then morning dress is called for. If both events take place after that time then evening dress is appropriate.
However, when the ceremony is scheduled for the afternoon and the reception for evening then convention dictates that the daytime dress code applies even if it means that morning dress will be worn well into the night. Alternately, the groomsmen can change from morning dress to evening wear between events. Since wedding guests no longer sport formal day wear in North America invited males may wear the same dark suit to both functions unless “Black Tie” is indicated on the reception invitation. In that case they too will be required to change into tuxedos before the evening’s festivities. (Ladies would also be expected to slip into something more formal as it would be déclassé to wear cocktail dresses to a daytime function.)