There is quite a bit of etiquette and tradition surrounding weddings, and it is not only meant for the bride and groom. The wedding guests also have their part to play, and if they abide by the rules governing good taste and decorum, the wedding will end up being a more pleasant event for everyone present. These are a few simple things to keep in mind when you are invited to a wedding.
The number one thing that a guest should remember is that it is in fact an honor to be invited to a wedding, not a chore. Nor should a wedding invitation be seen as an attempt at bribery. It is actually an invitation to participate in a very momentous event, no matter what the cynics see. Because of this, wedding invitations should be accepted, unless you have a pressing conflict, such as another wedding on that date, or if you can not afford to travel to a far-flung destination wedding. Dinner at the White House would be a good reason to decline a wedding invitation; an interesting movie being released on the wedding date would not. Either way, do your hosts the courtesy of accepting or declaring the invitation as soon as is practical.
Assuming that you have accepted the invitation, the next thoughts on your mind are likely to be your wedding attire and the wedding gift. Etiquette does hold that a guest has up to a year after the wedding to send a gift, but it is really much better to send it before the wedding while the excitation of the event is at its highest. Contrary to what many people believe, you should not bring a gift to a wedding as you would to a birthday party. The only exception would be if you are bringing a check, which you could discreetly slip to the groom at the reception with a handshake and a heartfelt "Congratulations!".
The dress codes for weddings have relaxed just as they have for everything else, but there are still a few basic principles that it would be wise to obey. The number one rule is that only the bride should wear white. A female guest who wears an all-white ensemble will certainly be the recipient of the shocked stares of people who think she is trying to upstage the bride, even if that is not her intention. An all-black outfit used to be equally taboo, although you do see a quite a bit of black worn at weddings these days. Just save the solid black dress for an evening wedding, please. In general, the female guests should wear pretty dresses with exquisite jewelry to an afternoon wedding, and can break out their fancy cocktail dresses and more elaborate wedding jewelry after 6pm. The men will wear suits during the day (or a blazer and trousers to a semi-formal wedding), and tuxedos or suits in the evening.
Many of the other rules of wedding guest etiquette are the same as those that would apply to being a guest at any type of party. Do not be the first one to come or the last one to leave. Do not drink yourself under the table (unless you are at a Russian wedding, in which case it is staying sober that is considered rude). Remember to thank your hosts for having you, and at the same time you may offer best wishes to the bride and congratulations to the groom (it is technically improper to congratulate a bride, as in the past it was thought to translate to congratulations on finally snaring yourself a husband!).
There are a few other things that a wedding guest can do to add to the general festival mood of the occasion. One is to make a point of seeking out anyone who seems to be alone and engage them in conversation or invite them to the dance floor. Another is to remember that a wedding is a happy event, and your topics of conversation should be equally lighthearted. This is not the time to bring up your recent divorce or any other sad news; if you can not think of anything else to talk about, it is always safe to compliment the beauty of the bride and the delicious dinner.
With these things in mind, you will be a model wedding guest, and yours will be a welcome presence. It is truly a delight to the newlyweds to feel that all of their guests are having a wonderful time at their wedding. And before you leave the house for the ceremony, remember that the most important thing to bring is a smile on your face (and maybe a few tic-tacs).