The Hijri calendar runs entirely on the lunar cycle, with no leap month or seasonal correction. A Hijri year is approximately 354 or 355 days – about 10 to 11 days shorter than the 365-day Gregorian solar year. As a result, every Islamic date arrives roughly 10 days earlier in the Gregorian calendar each year, and over a full cycle of approximately 33 years, any given Hijri date will pass through every season. Some see a connection with the 99 names of Allah, confirming a “sacred harmony” to the calendar’s movement that is independent of the solar cycle.
This shift is a deliberate feature of the calendar, not a defect. Because Islamic worship is tied to lunar months rather than fixed seasons, every Muslim experiences Ramadan, Hajj, and the major holidays in different climates over the course of a lifetime – sometimes in the long days of summer, sometimes in the short days of winter. The equity of this rotation, across regions and across generations, is part of why the lunar reckoning is preserved unchanged in the Qur’an.
The pre-Islamic Arabs had at times attempted to “postpone” months (nasi’) to periodically add a month to the lunar year to align it with the solar seasons. The Qur’an condemned this practice directly in Surah at-Tawbah 9:37: “Indeed, the postponing [of restriction within sacred months] is an increase in disbelief,” declaring that the months must remain as Allah ordained them. The Prophet Muhammad (peace be upon him) confirmed this in his Farewell Sermon, declaring that “time has come back to its original state which it had when Allah created the Heavens and the Earth” (Sahih al-Bukhari 4456). By enforcing a strictly lunar year, he “severed the months from their relationship with the seasons and from their millennial link to the seasonal fertility cults” (The Prophet Mohammad: A Biography, Barnaby Rogerson, p. 111). This ensured that Muslim feasts would never be linked to pagan seasonal festivals like the winter solstice or the spring equinox.
Because the calendar follows actual moon observation, the start of each month can differ by one or two days between regions. The dates printed on this calendar are astronomical calculations – useful for planning, but not authoritative for ritual purposes. The actual start of Ramadan, Eid al-Fitr, Dhul-Hijjah, and the day of Eid al-Adha is determined by the moon sighting as announced in your local community or by the authority you follow.
Hadith of Kuraib
The absolute primary text used by scholars to justify that communities should follow their own local authority is a famous narration in Sahih Muslim 2412 known as the Hadith of Kuraib: A companion named Kuraib traveled from Medina to Sham (Greater Syria). While he was in Syria, the crescent moon of Ramadan was sighted on a Friday evening, and the governor, Mu’awiyah, as well as the local population, began fasting. Toward the end of the month, Kuraib returned to Medina and spoke with Abdullah ibn Abbas (the Prophet’s cousin and a premier jurist). Ibn Abbas asked Kuraib when they had sighted the moon in Syria. Kuraib replied, “We saw it on the night of Friday.” Ibn Abbas responded, “But we saw it on Saturday night. So we will continue to fast till we complete thirty (fasts) or we see it (the new moon of Shawwal).” Kuraib asked, “Is the sighting of the moon by Mu’awiya not valid for you?” Ibn Abbas decisively answered: “No; this is how the Messenger of Allah (ﷺ) has commanded us.” This text proved that even though Syria and Medina were part of the same Islamic state, Ibn Abbas rejected applying the Syrian moon sighting to the people of Medina because they belonged to different geographical horizons.
For practical purposes, the Gregorian dates on this calendar are accurate to within a day in most years. They are most useful for long-range planning – knowing roughly when Ramadan will fall in five years, or when Hajj season approaches – and least reliable in the immediate days before a major observance, when local sighting always supersedes calculation.
Because the calendar follows actual moon observation, the start of each month can differ by one or two days between regions. The dates printed on this calendar are astronomical calculations – useful for planning, but not authoritative for ritual purposes. The actual start of Ramadan, Eid al-Fitr, Dhul-Hijjah, and the day of Eid al-Adha is determined by the moon sighting as announced in your local community or by the authority you follow.