Anyone trying to book time off for Eid discovers a peculiarity: the date is not fully known in advance.
Because the month begins on sighting of the crescent, Ramadan may run twenty-nine or thirty days, and which it is may be determined only on the evening of the twenty-ninth. Communities watch for the crescent that night; if it is seen, Eid is the next morning, and if not, another day of fasting follows. This produces the familiar situation of two possible Eid dates being held open, and of the announcement coming the night before.
Governments handle it differently. Saudi Arabia has a moon-sighting committee whose declarations many follow; several countries use calculated calendars for civil purposes and sighted ones for religious ones; Turkey has used astronomical calculation since 1978. In Britain, different mosques follow Saudi announcements, local sighting, or a calculated criterion, with the predictable result.
The disagreement is old and is not treated as scandalous within Islamic jurisprudence, which has long accepted that people in different places may legitimately observe on different days. It becomes a practical grievance in minority contexts, where employers and schools want a date in advance and where a community split across two days has less standing to ask for one.
Campaigns for a unified calculated calendar recur and have not succeeded. The obstacle is not technical, crescent visibility is predictable to high accuracy, but jurisprudential, turning on whether the classical requirement was the sighting or the knowledge.