According to quite a lot of religions and various superstitions, the heavens opening on your big day is a good thing (only as long as you have yourself a good supply of umbrellas, hint, hint.)
These are just a few of the reasons for rain to bring good luck:
No more tears - Rain on your wedding day is thought to represent the last tears that the bride will shed as she now enters a happy blissful life.
Cleansing & renewal - Rain washes away dirt and leaves a fresh, clean scent when the sun returns, so rain on your wedding day signifies the washing away of bad memories, tough times or sadness, giving you both a fresh slate on which you can metaphorically write down your happy future together.
Fertility - Rain is thought to signify that you will have children. This belief came from our agricultural past, because as gardeners know, when the rains arrive the ground becomes fertile, allowing for the new growth of plants and vegetation, and in some hot lands there are insects and small animals that lie dormant underground until the rains appear. These creatures magically appear almost overnight making it seem that the rain has created life.
A lasting marriage - A knot that becomes wet is very hard to untie, therefore when you 'Tie the knot' on a rainy day your marriage is supposedly just as hard to unravel. We can thank Hindu traditions for this belief.