People celebrate that they are getting married. The ceremony is usually called a wedding.
In Western cultures, a wedding ceremony usually consists of the couple exchanging vows and rings, following which the couple kiss. There is often a party afterward called a reception. During the reception, the couple often cut a cake, and have their first dance together. The man getting married is called the bridegroom, and the woman getting married is called the bride. Within Western cultures, the bride usually wears a white dress, because white symbolizes purity, and sometimes wears a thin piece of white cloth called a veil.