On a late Friday afternoon, Lamu men and boys in traditional Islamic dress gather to pray at Lamu's Islamic graveyard before setting out in procession to celebrate Maulidi, the Prophet Mohamed's birthday. This is a major event in Lamu and attracts pilgrims from all over East Africa and sometimes farther afield. The celebrations last several days and each year, take place ten days earlier than the previous year's celebrations. Tambourines are played in mosques on the island but they are seldom used in Islamic culture elsewhere. Situated 150 miles north-northeast of Mombasa, Lamu town dates from the 15th century AD. The island's importance lies in the fact that it has the only certain source of sweet groundwater in the entire district. The town was declared a World Heritage Site by UNESCO in 2001 because it is the oldest and best preserved Swahili settlement in East Africa. Moreover, its architecture and urban development graphically demonstrates the diverse cultural influences that came together over many centuries to produce a distinctively local Swahili style and culture.