The mothers' dresses should be somewhat conservative, and not longer or more formal than her daughter's dress. Usually the mothers wear a neutral color or a lighter shade of the bridesmaids' dresses.
Below you will find guidelines for the proper mothers' day and evening attire.
What Not To Wear
Mothers should not wear white dresses to the wedding. I have seen several mothers wearing white dresses that closely resembled wedding gowns - this is VERY tacky!
Speak with the bride before wearing a black dress. While black is a standard color for evening wear, it can stir up controversy when it comes to weddings. In the past, if either mother wore black it meant that they did not approve of the wedding. Today, black is more accepted but may still cause a flurry of gossip. Ask the bride how she feels about black before you purchase a black dress.
Dressing Guidelines for Mothers
For a very formal day or evening wedding the mothers should wear floor-length gowns and these gowns should NOT be white. Wear heels that are appropriate for the dress but try to select comfortable shoes. Coordinate your accessories but leave the watch at home! For a formal wedding the mothers should wear corsages or carry small bouquets.
For a semi-formal day or evening wedding the mothers should wear a street-length dress, again, it should NOT be white. Heels and accessories should coordinate with the dress. For a semi-formal wedding the mothers should wear corsages.
For an informal day or evening wedding the mothers should wear a suit or nice "Sunday" dress, again, it should NOT be white. Or, if the informal wedding is being held at the beach or some other fun or unusual location, the mothers should dress appropriately. Heels and accessories should coordinate. For an informal wedding the mothers should wear corsages.