Hey Canberra what Tomcat is saying is very true for starters you need to plan it 12 months in advance so people can organise leave, there will be those that say yes now and then last minute change thier mind and there will be those that decide to go last minute.
First you need to set a date then poll to find a location or state it to be held but most importantly remember you don't have to do it by your self if nessary put forward the idea of someone suggesting thier local location then have a few help that person organise the details