The Islamic calendar
The Hijri calendar is a lunar calendar of twelve months, counted from the Prophet's migration (hijra) from Makkah to Madinah in 622 CE. Allah says: 'Indeed, the number of months with Allah is twelve' (Surah at-Tawbah 9:36). A lunar year is about 354 days, eleven days shorter than the solar year, so Islamic dates move earlier through the seasons each year.
Why dates can differ by a day
This tool uses the Umm al-Qura calculated calendar, used officially in Saudi Arabia and by most apps. But the start of each month is traditionally confirmed by sighting the new moon, which can fall a day before or after the calculation depending on your location. For Ramadan, the two Eids, and Hajj, follow your local moon-sighting authority.
The twelve months
The months are Muharram, Safar, Rabi al-Awwal, Rabi al-Thani, Jumada al-Ula, Jumada al-Thani, Rajab, Shaban, Ramadan, Shawwal, Dhul-Qadah, and Dhul-Hijjah. Four are sacred - Dhul-Qadah, Dhul-Hijjah, Muharram, and Rajab - in which fighting was traditionally forbidden.