First, you should probably decide on a special day that you want to have your big day? Sound easy? Keep in mind, you will want people in attendance so either stay away from major holidays or tack on to long weekends to make it easier for your guests and bridal party to attend/participate.
Second, determine the place and if the place will be available on the special day you chose. Obviously if there is a conflict, you will either have to change the venue or change the date.
You will need that information in order to create your bridal party! They need to make sure they can be available to walk you down the aisle and celebrate with you.