This week marks the first week of the Muslim holy month of Ramadan.

In Islam, it is believed that God revealed the first verses of Islam’s holy book, the Quran, during Ramadan. During the holy month, Muslims fast from dawn to sunset every day.

Fasting means that Muslims cannot drink or eat anything, smoke, or engage in sexual activity during daylight hours. While Ramadan is always a month long, the start date moves back around 11 days each year. This is because Ramadan is based on the Islamic lunar calendar.

While the month can sometimes be physically or mentally taxing, especially during the summer months, it is meant to be a time of reflection and calm. Not all Muslims are mandated to fast, however–children, the elderly, pregnant women and people who are ill are excused from fasting.

At the end of Ramadan, Muslims celebrate with a three-day festival called Eid al-Fitr.