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Dressing for daytime weddings can be so much more comfortable -- not to mention inexpensive -- than for evenings. But "semi-formal" attire can be confusing; it encompasses the vague territory between full-on ball gowns on the "formal" side and casual clothes on the other. The rule of thumb is to dress as fancy as you can without looking like your partner should be wearing a tuxedo. Keep your hems at the knee, and err on the side of formality.
Traditionally, semi-formal dresses are knee-length, though you can safely wear one that reaches 1 inch above the knee (not any shorter than that) or one that is tea-length (a couple of inches above the ankles). Think "cocktail dress," but don't wear a dress that's so sexy and flirty that it would be more appropriate for clubbing than for celebrating a friend's daytime wedding.
You can definitely wear separates to a semi-formal event, as long as the pieces are dressy. Try a full, long satin skirt with a cashmere top, or a chiffon skirt with a lace camisole. An elegant brocade suit and matching knee-length skirt would be perfect. Pantsuits are also fine, as long as they're ankle-length, stylish and made of materials such as silk or brocade.
The "semi" in "semi-formal" is not permission to wear denim! This also isn't the time to wear wool, which isn't considered dressy enough (except cashmere). When choosing an outfit, look for taffeta, chiffon, sateen, gossamer, silk, or satin. In the winter, velvet and velour are both appropriate.
Shoes and Accessories
Traditionally, semi-formal attire calls for dress heels, but you can get away with dressy flats or strappy sandals, as long as they're color-coordinated with the rest of your outfit. Go ahead and wear jewelry, but keep it simple: stud earrings or very simple long ones; a pearl or chain necklace with few embellishments; an elegant bracelet and one or two rings. Your handbag should match your outfit. For daytime weddings, you can also wear a hat, as long as it's not too flashy.