Videos
Weddings are a joyous occasion. Take your opportunity to celebrate with the happy couple, meet some new people, and reacquaint with old friends. Congratulate the parents of the bride on how beautiful their daughter looks or how lovely the wedding has been. Bring your card with you (you did get them a card, right?), but have the gift mailed to their house (with the gift receipt). Be respectful and most of all, HAVE SOME FUN. Your outfit is the last thing on anyone's mind.
This guide offers some tips and ideas on how you can make the most of your outfit on somebody else's special day. It will be mostly American-centric due to my lack of knowledge concerning International/British customs. Please feel free to chime-in. Again, this is advice, nobody on r/mfa knows you, your family, friends, or the couple getting married like you do, asking them for advice is a good idea too.
TL;DR: Choosing appropriate attire for a wedding is quite simple:
Follow any stated or implied dress code. Ask someone close to the bride or groom if you're unsure.
Wear something appropriate for the occasion. Your outfit should be smart and festive, it should NOT be golfing, clubbing, business, or funeral attire.
Don't overdress or attempt something too flamboyant. You'll want to wear clothing you're comfortable with and you don't want to accidentally upstage the bride or groom.
BASIC WEDDING GUEST ATTIRE
If you're completely clueless at dressing yourself and you want to follow very specific rules concerning your outfit follow these guidelines to form something competent and presentable.
Wear your smartest/nicest outfit in simple colors and limited patterns.
Wear what you already own (or at least what's passable). All clothing should fit properly, be in good condition, be washed and ironed. DON'T buy something brand new to impress people unless you WANT to buy something or you NEED to buy something. Remember, nobody is too concerned with what you're wearing anyways, wedding days revolve around the happy couple.
Attire #1: Wear this if possible
A short album #1
Suit - Solid navy and gray are the safest colors, try to avoid black or pinstripes.
Shirt - White or light blue dress shirt - point collar, barrel or french cuffs.
Shoes - Black dress shoes and belt, brown is acceptable. Cap-toes or semi brogues preferred.
Tie - Silk tie with texture, repeating motif, or simple pattern such as stripes, pindots, checks. Avoid solid satin, gaudy colors, and kitschy patterns (like repeating boats or cats).
Pocket square - optional - white linen in a simple TV fold is safest. DON'T wear a matching tie and pocket square set.
Alternate attire #2: Assuming you don't have access to a suit
A short album #2
Jacket - Navy blazer - wool is preferable, cotton is acceptable.
Pants - Khaki or gray dress pants, wool is preferable over cotton.
Shirt - White or light blue button-up shirt - point collar or button down, barrel cuffs.
Shoes - Black or brown dress shoes and belt. Brogues and loafers are acceptable.
Tie - Silk tie with simple pattern or texture. Stripes, checks, pin-dots are good.
Alternate attire #3: Assuming you don't have access to a suit or a blazer and you're UNDER THE AGE OF 21. If you're older than 21 you might consider beginning your semi-formal wardrobe. It'll get used more frequently as your friends or family get married or as you enter your professional career.
A short album #3
Shirt - White, light blue, or white based shirt with blue pattern (checks or stripes) - Shirt should fit well, point or button down collar, barrell cuffs.
Pants - Khaki or gray pants, cotton is fine.
Shoes - Brown dress shoes and belt. Brogues, bluchers or loafers are acceptable. NO SNEAKERS.
Tie - optional - Repp stripes or knit silk in a navy or red base. Nothing wider than 3 1/4 inches, nothing skinnier than 2 1/4 inches.
If you're a bit more accomplished at dressing yourself and want to select something appropriate but individualized, ask yourself the following questions to help pinpoint what you might wear.
I get it, you want to wear something to showcase your new talents at dressing but you're not exactly sure how to create the perfect outfit. As you're thinking about your clothing, you should be considering the wedding that you're attending and the guests in attendance. Dress for your specific situation.
How formal is the wedding? What is the stated dress code?
Most weddings will have guidelines for a dress code in the invitation or they will imply somehow to the formality of the occasion. Your job is to dress appropriately to match what the bride and groom specify, even if it's not directly stated. If you don't know what the dress code is ASK SOMEONE WHO KNOWS THE BRIDE OR GROOM. They'll be happy to tell you what, if any, the dress code stipulates. ALWAYS FOLLOW THE DRESS CODE, even if it's something like tuxedos during the daytime or aloha shirts in a landlocked state, it's their wedding day, not yours.
More formal attire includes more conservative color choices with suiting and shirting, proper dress shoes and a wedding tie. Less formal attire includes opting for a jacket over a suit, more casual fabrics, colors in your shirt and tie, and casual leather shoes such as bucs, brogues, or mocassin styles.
When is the wedding? What time of day and what season?
Nighttime weddings are generally more formal than daytime affairs and they're better for darker color palettes. Tuxedos should ONLY be considered for evening events (6:00 pm or sunset whichever comes first), and that's only if the dress code allows them. DO NOT WEAR A TUXEDO DURING A DAYTIME WEDDING. Daytime ceremonies suggest less formal outfits and brighter colors for both suits and accessories. Tan and light gray suits are much easier to wear in daylight. For weddings that begin in the daytime and end at nighttime, don't worry too much about outfit changes, you might consider bringing an extra layer for warmth.
Use the season to dictate your color choices and your material choices. Pay attention to the local weather forecast so you can dress for proper conditions and temperatures. Heavy fabrics such as tweed and flannel are better in fall/winter whereas tropical weight wool, linen, or seersucker are best in the summer. Seasonal footwear can help distinguish your outfit from other guests with dress boots in winter or bucks/loafers in the summer. Color changes in our environment are great indicators of the color palettes you might consider. Basic seasonal colors are as follows:
Spring = pastels, pinks, light blues, light yellows
Summer = bright colors, oranges, reds, blues
Fall = earth tones, fall colors, browns
Winter = deep colors, burgundy, dark green, plum
Where is the wedding located? Geographic location? Venue locations?
Geographic location plays a large role in determining acceptable outfits due to changes in climate, culture, and people. Texas is known for cowboy boots and decorative belts, New England is preppy, and large cities may have more forward thinking styles. If you're travelling to a region with different style than your own you have three options:
Dress in your own style - Where you come from is who you are, no need to change. Wear that seersucker suit on the west coast or that bolo tie in New York. Your family/friends would expect nothing less.
Dress in the style of the wedding's location - If you've always wanted to experiment with a style not popular in your region, this is the perfect opportunity. This should be kept within reason of course, don't wear a costume and keep it in good taste.
Dress in a conservative, traditional style regardless of location - This option is the safest and likely where most wedding guests will end up. There's nothing wrong with a man in a uniform (read: navy or gray suit and wedding tie).
Weddings are often planned around their venues. Grand hotels are more formal than small banquets which are more formal than outdoor picnics. Indoor weddings tend to be more formal that outdoors. However, don't assume that all outdoor weddings are strictly casual. Try to imagine the venue you're headed to and what the typical attendee might be wearing. For example a wedding near the ocean could be interpreted as nautical-themed, whereas a wedding on a farm might be more workwear. I'm not advising you to wear a costume such as denim overalls to a farm wedding, but you might be more inclined to wear flannel or boots.
Selecting footwear is also related to practicality. You don't want to be wearing leather soled shoes to walk on wet-grass and you might not want dress boots if you're in a standing on a beach.
I ran over the limit, continues in the comments.