Weddingbee boards

a quick google search brings up differing viewpoints.  according to some posts, official wedding etiquette people at stationer's believe that 6pm is the official start of the evening, so if you are stickign with the rules, five o'clock would be the afternoon.

the google search also pulled up a weddingbee post on the same topic:

http://boards.weddingbee.com/topic/5-oclock-in-the-afternoon-or-5-oclock-in-the-evening

if your event starts at five o'clock, with other events to follow, evening makes more sense to me (personally speaking).  obviously, there have been at least a few people struggle with the question, so chances are good that your guests may not be up on their etiquette, so you may be able to go either way, depending on the tone you'd like to set.  good luck!