The first Eid was celebrated in 624 CE by the Prophet Muhammad with his friends and relatives after the victory of the battle of Jang-e-Badar.
The festival begins when the first sight of the new moon is seen in the sky.
Muslims in most countries rely on news of an official sighting, rather than looking at the sky themselves.
The celebratory atmosphere is increased by everyone wearing best or new clothes, and decorating their homes.
There are special services out of doors and in mosques, processions through the streets, and of course, a special celebratory meal - eaten during daytime, the first daytime meal Muslims will have had in a month.
Eid is also a time of forgiveness, and making amends.