Before the Wedding
RSVP as soon as possible
Once you have your invitation in hand, make note of the RSVP date, and send your response as far in advance as you can. Better yet, if you already know you're attending, turn around and drop the RSVP card in the mail immediately!
Respect the guest list
If you're single and the bride and groom would like you to bring a guest, your invitation will say "and guest," or they will call you ahead of time to see whom you'd like to bring and address the invitation accordingly. Children are invited if the invitation lists their names or says "and family." For heaven's sake, don't call the couple and ask to bring people they didn't invite—that's uncomfortable for everyone.
Utilize the resources the couple provides
Read the couple's wedding website thoroughly before asking questions about travel or schedules. If you really can't find information you need, try contacting the mother of the bride or a member of the bridal party before asking the bride or groom directly, especially in the weeks and days leading up to the wedding.
At the ceremony
Be mindful of the dress code
This will often be specified on the wedding website, but if not, make an educated guess using the invitation, venue, and time of day as guides. A sundress and wedges are probably fine for an afternoon barn wedding, but an evening ballroom wedding requires at least a cocktail dress. When in doubt, dress up. Many guests look forward to weddings as opportunities to look their very best, and we love this tradition! Additionally, pay special attention to the attire guidelines for ceremonies held in houses of worship.
Don't wear white
Unless the bride and groom expressly ask you to wear white, don't do it. Some people consider this rule out of date, because no one will mistake a guest for the bride, but even if you know the bride won't mind, it's very likely that other guests will find your choice of attire rude or disrespectful. With all of the beautiful and colorful options available today, leave the white sundress at home for this one.
Arrive at least 15 minutes (and up to a half hour) before the ceremony
Give yourself time to park your car, pick up your program, and take your seat without even a chance of running into the bride before her walk down the aisle! Weddings are one occasion where there is no such thing as fashionably late. If you do arrive after the specified time despite your best efforts, an usher may be able to direct you inside at an appropriate time. If not, wait for a "louder" part of the ceremony, such as a hymn, before quietly seating yourself in the back.
Put your camera and cell phone away (on silent)
Many couples now specifically request "unplugged" ceremonies, but even if they don't, resist the urge to snap pictures at the ceremony. Not only can a clicking camera be distracting to other guests, but it can also make the professional photographer's job difficult.
At the Reception
Don't attend the reception if you didn't go to the ceremony
The ceremony is the most important part of the wedding, and the reception is a celebration of that—not just a party. If you don't want to attend both, it may be best to stay at home.
Sit in the appropriate seat
If the bride and groom have specified table numbers and seating assignments, sit where they ask you to. They have probably spent several hours laboring over whom to place where, and who knows what connections you'll make with your fellow guests!
Be mindful of social media
While couples almost universally prefer to have their ceremonies be social media free, there's not a one-size-fits-all guide for the reception. Some couples make it clear (often with a hashtag sign) that they welcome guests sharing photos, posts, or tweets, while others would prefer not to have their photos splashed over every newsfeed. Follow the couple's lead, or let your best judgment of their personalities guide your sharing.
Enjoy the meal, get on the dance floor, sign the guest book, attempt to catch the bouquet, and wish the bride and groom well. They've worked hard to make their reception a wonderful evening for you as well as them, and they'll be thrilled to see you having a good time.
After the Wedding
Send a thank-you note
While your duties as a guest largely conclude when you head home from the reception (after the cake is cut, of course!), there's one surefire way to earn yourself a spot in the hall of fame: Send a thank-you note. A handwritten message to the bride and groom and/or to their parents, if they're hosting, can mean so much after a year or more of intense planning.