Is expiation obligatory upon one who ate, thinking that the day was not part of Ramadan, then it became clear to him that it was the first day of Ramadan?

Whoever realizes during the day that it is Ramadan (definition) must abstain from eating and drinking for the rest of their day and make up that day. This is the view of the majority of the three Imams, contrary to Sheikh al-Islam Ibn Taymiyyah, who holds that making up the day is not obligatory. Delaying the making up of a missed day without a valid excuse until the next Ramadan begins necessitates feeding a poor person. Intentionally breaking the fast after knowing that the month has begun is a grave sin that requires repentance, and expiation (kaffara) is not obligatory along with making up the day.