Rain on one's wedding day is viewed by many cultures as a positive omen, portending fertility, renewal, cleansing and unity. Some, however, view a rainy wedding day as having a negative connotation, symbolizing that the bride is likely to shed many tears during her married life.
The notion that rain on a wedding day is a symbol of fertility is related to the fact that rain waters the ground, allowing plants to grow. The idea that rain signifies renewal relates to the rain washing away all the sorrow and struggle in one's life and providing the ability to start fresh; rain's cleansing qualities wash away sadness and hard times. The expression "tying the knot" is derived from rainy wedding day lore. Since a wet knot is more difficult to undo, rain on a wedding day means the marriage is likely to remain strong.