No bride wants to have rain on her wedding day. I have heard all the old sayings, including, "It's bad luck because it's an indicator of all the tears you'll cry during your marriage" and the Chinese tradition that welcomes rain as washing away worries and offering a new beginning.
Regardless of the superstitions surrounding rain and weddings, inclement weather can interrupt many aspects of your day, from outdoor picture taking to trying to get from the church to the car without getting soaked.
It's incredibly important to make contingency plans for the weather, especially if a large part of your day is going to be spent outdoors. If you're planning on having lots of photos taken outside, be sure to have a backup place where you can also take pictures. If the ceremony and/or reception are taking place outdoors, have a tent or pavilion that can be used in case of a sudden downpour. It may cost a little more and take some extra planning, but it will be worth it if Mother Nature decides not to cooperate.