Living in Southern California the weather is pretty mellow. Though, sometimes in the fall the wind picks up and the rain does fall. If using standard black painters plastic and PVC pipes, you need to make sure to cut a few slits in the walls if there is any wind what-so-ever.
Regarding the roofing,
I would suggest that you wait until just before the event to attach your ceilings. Even if there is no rain, your roof will give a little and begin to droop. We usually wait until about a week before to attach our plastic roofs.
If it is absolultey necessary that you attach your roofing prior to the 1 week mark, and your area does recieve heavy dew or rain, poke several small holes in the roof for water drainage.
I hope this was somewhat helpful.