Put simply, the Odalan is a six-month anniversary of a temple family or village temple. Bali using a calendar month, not the solar calendar. Well once every six months Balinese calendar is a temple will be celebrated with a ceremony called the Odalan. Odalan could also mean the day of birth or the creation of a temple / shrine at Bali.
Odalan identical to the activities of a religious ceremony in a temple. There will be plenty of activities to the left of a Odalan. Activities can berlang Odalan until a month depending on the type of a temple / shrine.
Many people will be involved in Odalan, depending on how extensive range of a temple. Suppose the Odalan held in Besakih temple, the Odalan that involved in the whole society is Hindu Bali.