How Are Islamic Holidays Determined?
Unlike the Gregorian calendar which is solar (based on the Earth's revolution around the sun), the Islamic (Hijri) calendar is strictly lunar (based on the phases of the moon). A lunar month is either 29 or 30 days long, meaning a full lunar year is about 354 or 355 days. Because it is 10 to 12 days shorter than the solar year, Islamic events migrate backward through the seasons. Over a 33-year period, Ramadan will be experienced in winter, autumn, summer, and spring!
The Sacred Months (Ashhur Al-Hurum)
Out of the 12 Islamic months, Allah has designated four as sacred: Muharram, Rajab, Dhu al-Qi'dah, and Dhu al-Hijjah. During these months, the reward for good deeds is multiplied, and the punishment for sins is also more severe. It is highly recommended to increase fasting, charity, and prayer during these specific periods.