From Encyclopedia Britannica:

Eid al-Fitr, one of the two main festivals in Islam. Eid al-Fitr marks the end of Ramadan, the holy month of fasting in which Muslims believe God revealed the Qurʾān to Muhammad through the angel Gabriel (Jibrīl). It is celebrated during the first three days of Shawwāl, the 10th month of the Islamic calendar (though the Muslim use of a lunar calendar means that it may fall in any season of the year). Eid al-Fitr is a time of official receptions and private visits, when friends greet one another, and presents are given.