Planning and preparing for the ceremony can be the most stressful part of weddings, and you don't want that stress to spill over into the event. The sooner you take care of the details, the better. Coordinate with the couple and get the answer to these questions…
Make sure to block out the time. Put it in your calendar, phone, post-it on the fridge, etc.
It may be your backyard, it may be a Comicon convention. Either way, the venue can inform the tone of the ceremony. Additionally, it helps to know where you will be speaking.
Maybe you have to book plane tickets. Or catch a train. Or do both. Figure it out, lock in in, and give yourself a pat on the back because travel is stressful enough without last-minute scrambling.
The way that we dress sends important social cues, and helps to set the tone of the ceremony. Find out what the preweds have in mind, and if they don't know, collaborate and maybe you'll come up with something interesting or fitting together.
This takes care of the important logistics. Getting these points out of the way early will help to establish the tone you want to set when you begin writing the ceremony.