In the Jewish faith---as in every other faith---a wedding is a happy occasion. The ceremony itself can be moving and beautiful, and Jewish wedding receptions are well-known for being jubilant celebrations. If you are a non-Jew and have been invited to a Jewish wedding, don't worry. In most respects, it will be like any other wedding you have attended. If you keep a few etiquette tips in mind, you will show respect for your hosts---and have a great time.
What to Wear
Most Orthodox Jewish weddings are formal or black-tie affairs, so you should dress accordingly. For a "dressy casual" wedding, you should wear what you'd wear to synagogue (tie for men, skirt or dress for women). If the invitation reads "Please dress modestly," this means hemlines should be below the knee, sleeves should be long and necklines should be high. It's perfectly acceptable for a woman to wear black to a Jewish wedding, but if she's wearing a dress that is low-cut or exposes her shoulders or arms, she should bring a shawl or a sweater for the ceremony. A man will probably be asked to wear a yarmulke (a skullcap) in the synagogue. You needn't bring your own, though; it will be provided.
In the Jewish tradition, wedding gifts are sent to the bride's house in the weeks before the wedding. If you do bring a gift to the wedding or reception, though, many people have gift tables set up to accommodate their friends who aren't familiar with this custom. Engaged couples have often registered at a major department store for things like crystal, silver and china. Ask them or one of their parents if you'd like to give them something from their registry. Money is a very common Jewish wedding gift as well. Since the number 18 is symbolic of "life" in this culture, the amount of money given is often a multiple of 18.
Where to Sit
Depending upon the degree of orthodoxy the families observe, you may be seated with your escort on the bride's side or groom's side or segregated by genders. Modern Orthodox Jews often have mixed seating but not mixed dancing. When in doubt, watch what the others around you are doing and don't be afraid to ask questions. Guests are held in high regard in the Jewish tradition, and people will be glad to help you out by explaining anything you don't understand.
During the Ceremony
Jewish wedding ceremonies take place under a canopy called a chuppah, which can either be freestanding or held up by four people. You don't have to be Jewish to perform this task, and it is an honour to be asked. If you get the chance to be a chuppah holder, all you have to do is keep the pole in your corner upright during the wedding. You may also be asked to participate by reading one of the Seven Blessings (in English if you don't speak Hebrew), but you will be asked to do this well in advance of the wedding.
At the Reception
After the wedding, the newlywed couple spends about 20 minutes alone together, eating and speaking quietly with one another, and it is very bad manners to interrupt them during this time. Follow the rest of the guests to the reception hall and find your seat. The meal at the wedding reception is considered to be part of the ceremony and will probably be kept strictly kosher, so don't begin to eat until someone has said a blessing over the bread. There will probably be dancing at the reception---though depending upon the sect, genders may not dance or even sit together---and if you are a well-muscled man, you may be asked to help carry the bride and groom on chairs at shoulder-height around the dance floor.