Bangladesh has numerous public holidays, including national memorial, religious and secular holidays of Bengali origin. The Bengali traditional calendar, known as Banggabda is the national and official calendar in Bangladesh. The holidays are celebrated according to Bengali, Islamic or Gregorian calendars for religious and civil purposes, respectively. Religious festivals like Eid are celebrated according to the Islamic calendar whereas other national holidays are celebrated according to the Bengali and Gregorian calendar. While the Islamic calendar is based on the movement of the moon, it loses synchronization with the seasons, through seasonal drift. Therefore, some public holidays are subject to change every year based on the lunar calendar.