For women: Please don’t try to out do the bride. You want to look nice, but not like you’re going out to the club either. No revealing hemline or busting out of any sort. You will probably want to wear a dress that is at least touching the top of your knees, and don’t wear a long evening gown because it would be too formal and you would stick out like a sore thumb (unless your invite informs you to wear something ULTRA formal.) Remember to pick a color other than white so you are not competing with the bride. Wear a sundress, cocktail dress (warm weather), or sweater dress (cold weather) at a daytime wedding.
For a night time wedding you want to wear darker colors, maybe a little black dress and if it is cold out, wear a sweater or cute jacket over top. You could also resort to wearing leggings with a sweater dress and another option other than a dress could be a pencil skirt and cute blouse top. If you’re hesitant about what kind of dress to wear, make sure you wear a dress that has straps so you can dance and move around without having to adjust the top of your garment the whole wedding.
Wear some comfortable heels/wedges/boots since you might have to stand for a long time or walk a lot depending on how close you can park to the venue, of if there is a buffet line at the meal, etc.
Wear pretty earrings or a nice necklace or both depending on how busy your outfit is. Do your makeup and hair however you want. I would suggest keeping hair and makeup simple so you aren’t having to go fix it every hour. I usually bring whatever purse to a wedding, but you might want something smaller like a clutch that has a powder compact, lip gloss, and some gum (you’ll probably socialize a lot and want to have nice breath). You can thank me later!
For men: For a day wedding Wear a dress shirt, with a sports jacket or blazer and some nice dress pants and dress shoes. Avoid light colors unless it is ultra formal in which you would wear a white tux. For a night wedding, a dark suit is appropriate. You probably want to shave your face. Comb your hair. That’s basically all I can think of. Easy enough!