More weddings occur in the late spring, summer and early fall months. You'll find better deals on the locales and vendors during the "off season" of late fall, winter and early spring.
Schedule your wedding on a Friday or Sunday, or for early in the day. This can help to cut costs, especially for the reception. You can still plan a wedding for decent weather even if you wait until after the traditional summer season. Take advantage of off-season prices in the early spring or autumn when the weather still can be fair.
Of course, while a wedding costs less during off-peak times, consider the practicality of where and when it takes place. A winter outdoor wedding in freezing temperatures or a Southern wedding during hurricane season makes for an impractical ceremony. Since weather is often a factor in off-season weddings, be sure that your site offers a Plan B for that outdoor terrace you wish to use for the cocktail party. Securing a separate party room or backup transportation in case of inclement weather makes for smart planning.