This is a Moslem holiday celebrated by all Moslem in a Moslem country.
The day is celebrated to show the gratefulness to Allah (God). All Moslem people go the mosque and pray together in the morning. When the prayer is over, they slaughter some animals such as cows, goats or sheep and distribute their meat to the needy people.
Idul Adha is celebrated on 10 Dzulhijjah (according to the Moslem calendar system).