Because the Islamic calendar traditionally depends on the physical sighting of the new crescent moon rather than on a fixed astronomical calculation applied uniformly everywhere, different countries and religious authorities can and regularly do declare the first of Muharram, and every other lunar month, on different Gregorian dates in the same year, depending on whether local moon-sighting committees report a sighting, whether they accept sightings reported from other countries, or whether they instead follow a pre-calculated civil calendar such as Saudi Arabia's Umm al-Qura calendar, used for administrative purposes even though religious authorities there also maintain their own moon-sighting procedures for religious observance.
This lack of a single global authority for lunar sighting means Muslims in different countries, and even different mosques within the same country in some cases, can begin the Islamic New Year, and more consequentially Ramadan and the two Eids, on different days from each other, a recurring practical inconvenience for a globally interconnected Muslim population that international Islamic organisations have periodically proposed standardising through calculated astronomical methods, proposals that have not achieved universal acceptance because a significant body of Islamic legal opinion holds that actual physical sighting, not calculation, is what the tradition specifically requires.