A woman should wear a lightweight, semi-formal, knee-length dress with minimal decoration to a March wedding, according to About.com. Pastel colors are always appropriate for spring daytime weddings.
If the wedding invitation states the dress code is black tie optional, a full-length dress is appropriate, as is a shorter dress with some beading or other decoration. A black-tie wedding calls for a full-length evening dress but not a ball gown. If the wedding is taking place in a church, any dress should be modest; strapless dresses should be covered up with a jacket, which can come off at the reception.