Daytime weddings usually call for dress that's a little less formal, but you should always go with the recommendation on the invitation. Some people have formal daytime weddings, so you should be prepared to rent a tux or find an elegant dress if your friends are having a fancy wedding in the middle of the day.
Formality is largely based on length, and daytime dresses tend to be shorter, whereas evening gowns are always floor length. One of the prettiest looks for a daytime wedding gown is the ballerina-length tea gown. These dresses usually have a belled skirt: in the summer they tend to be sleeveless, and the whole look is slightly retro and very charming. Guests will also wear shorter dresses, but probably not as fluffy. Look for a sheath that ends around the knee: this may be an ideal occasion for your favorite Little Black Dress and a pair of medium-heeled pumps.
Jewelry is toned down in the daytime, and gloves may be worn, but they will probably be wrist-length. Tiara-type hair ornaments are definitely out, unless you're the bride, and lots of sequins or beads, which may look classy in starlight, can look tacky in the bright light of day. Go simpler, and let the fabrics do the talking. Skip satin in favor of silk, which has less shine but more luster. If the weather is nice, a linen dress can be the epitome of understated elegance.
