The use of the lunar calendar in Islam is a matter of divine legislation, rooted in the Qur'an and imbued with profound wisdom. Muslims follow the Hijri calendar because Allah has designated the cycles of the moon as the natural method for humanity to mark time, particularly for religious observances.

Allah says in the Qur'an: "They ask you, [O Muhammad], about the new moons. Say, 'They are measurements of time for the people and for Hajj.'" ([Qur'an 2:189](/quran/2/189)). Shaykh Ibn 'Uthaymeen explains that this verse establishes the new moons (ahillah) as a clear and universal sign for everyone, regardless of their level of education or access to technology. An unlettered person in the desert and a learned scholar in the city can both look to the sky to know the beginning and end of the months.

This contrasts with the solar calendar, the calculation of which requires specialized astronomical knowledge. The Islamic calendar's reliance on direct observation makes the practice of Islam, especially pillars like fasting in Ramadan and performing the Hajj, accessible and uniform for all Muslims everywhere.

Furthermore, Allah decreed the number of months to be twelve in His book on the day He created the heavens and the earth ([Qur'an 9:36](/quran/9/36)). The wisdom in the lunar year being approximately 11 days shorter than the solar year is that Islamic rituals rotate through all seasons. Over a 33-year cycle, Ramadan and Hajj will occur in winter, spring, summer, and autumn. This ensures that no single community is permanently burdened with fasting during long, hot summer days, nor is any community permanently given the ease of fasting during short, cool winter days. It is a manifestation of divine justice and mercy, distributing the experience of worship in different conditions among all peoples over time.