Dhu al-Hijjah can have 29 or 30 days, depending on crescent observation.
The Islamic calendar is lunar, and its months traditionally begin when the new crescent is sighted after conjunction. Because a lunar month lasts roughly 29.5 days, calendar months alternate between 29 and 30 days rather than following the fixed 30- or 31-day pattern used by the Gregorian calendar. Dhu al-Hijjah, like other Islamic months, therefore ends after either 29 or 30 days.
Dhu al-Hijjah is the twelfth and final month of the Islamic year. Its most important dates include the Day of Arafah on the ninth day and Eid al-Adha on the tenth. Hajj rituals begin on the eighth day and continue through the main pilgrimage period, traditionally ending around the 13th day.
Local moon-sighting decisions can make Islamic dates differ by a day between countries or communities. Some authorities use astronomical calculations or an official civil calendar instead of direct observation, but none of these systems makes Dhu al-Hijjah 31 days long. Its possible length remains 29 or 30 days.