Why do Muslims celebrate Eid on different days?

The root of it: the moon

Islamic months start with the new crescent. The Prophet ﷺ told people to start the fast when they see the new moon and to end it when they see it, and if the sky is cloudy, to fast thirty days (Sahih Muslim 1081). The question scholars have debated for centuries is: whose sighting counts, and does it have to be seen with the eye?

See How do I know which day Eid is?

The main approaches

• Local sighting: the moon must be seen in your own region. This is supported by a report from Ibn Abbas, who kept to the Medina sighting even though people in Syria had started Ramadan a day earlier, saying this is what the Prophet ﷺ commanded them (Sahih Muslim 1087). Many Shafi'i scholars hold this view.
• Global or wider sighting: a confirmed sighting anywhere (or in a nearby or reference country) counts for everyone. This is the view of many Hanafi, Maliki and Hanbali scholars, and some communities follow Saudi Arabia's announcement.
• Calculation: some councils, such as the Fiqh Council of North America, accept astronomical calculation of when the crescent can be seen, so they fix dates in advance. Others accept calculation only to rule out false sightings.

Each of these is a sincere attempt to follow the same texts. None of them is a "wrong religion" or a sign that someone doesn't care.

So which one should I follow?

For most new Muslims, the simplest and most widely recommended answer is: follow your local mosque or the community you pray with. The Prophet ﷺ said, "The fast is the day the people fast, the breaking of the fast is the day the people break their fast, and the sacrifice is the day the people sacrifice" (Jami' at-Tirmidhi 697). Many scholars take this to mean staying with your community matters more than winning the debate.

• Pick one source and stick with it for the whole of Ramadan and Eid, so you don't end up fasting 28 or 31 days.
• Don't fast on a day your community celebrates Eid, since fasting on Eid is not allowed (Sahih al-Bukhari 1990).
• Be relaxed about friends who celebrate a day earlier or later. Say "Eid Mubarak" to them on their day too.

Explaining it to family or work

A simple line works: "Our holidays follow the moon, and different communities confirm it in slightly different ways, so it may be one of two days." For school, see Can I miss school for Eid?