Muslims have two major celebrations every year; Eid-ul-Adha and Eid-ul-Fitr. They are very different festivals with different features and meanings.
Eid-ul-Fitr is celebrated at the end of the month of Ramadan, the 9th month of the Islamic calendar, where Muslims across the globe fast from sunrise to sunset and purify their actions by donating generously to charity and giving up bad habits.
The first Eid festival in 624 CE was established by the Prophet Muhammad (pbuh) and he celebrated with his friends and relatives after the victory of the battle of Jang-e-Badar.
Each year, the festival will begin when a new moon is sighted in the sky, although Muslims in most countries wait for an official sighting to be announced rather than scanning the skies themselves.