Wikipedia
Eid ul-Fitr or Id-Ul-Fitr (Arabic: عيد الفطر ‘Īdu l-Fiṭr), often abbreviated to Eid, is a Muslim holiday that marks the end of Ramadan, the Islamic holy month of fasting. Eid is an Arabic word meaning "festivity", while Fiṭr means "to break the fast" (and can also mean "nature", from the word "fitrah") and so symbolizes the breaking of the fasting period.
Eid ul-Fitr starts the day after Ramadan ends, and is verified by the sighting of the new moon.