Answer: Eid al-Fitr and Eid al-Ad'ha.
This is mentioned in the Hadīth of Anas who said: "When the Messenger of Allah (may Allah's peace and blessings be upon him) came to Madīnah, the people had two days on which they used to play games. He asked: 'What are these two days?' They said: 'We used to play on them in the pre-Islamic period.' Thereupon, the Messenger of Allah (may Allah's peace and blessings be upon him) said: 'Allah has given you something better than these two: the day of sacrifice (Eid al-Ad'ha) and the day of the breaking the fast (Eid al-Fitr).'" [Narrated by Abu Dāwūd]
All festivals, other than these two Eids, are counted as religious innovations.