Idul Fitri is the Islamic holiday that marks the end of the month long fasting of Ramadan. Many Indonesians from Java will return home for the holiday and a large number of domestic tourists will visit Bali for the holiday.
Police will be monitoring the massive movement of people leaving and entering Bali through the port of Gilmanuk where ferries connect Bali with Java. Not only will there be a high chance of traffic accidents, police fear terrorists might target the holiday period to enter the island of Bali.
Generally a good time to avoid travel if possible and many government offices, including immigration, are closed during the period.