wow that is lovely...i have no idea really to be honest...if it was me i would probably do it between the meal and the first dance maybe just before or just after the speeches (if you are having them)...my only reasoning is that you want them to have peoples full attention but I wouldn't want to do it once the party/disco/band whatever you are having has started because it might break up the atmosphere of people dancing etc?? sorry im not sure of thats helpful? I haven't been to a wedding since I was a kid so i might be talking rubbish! :)