I witnessed Eid with ‘Umar ibn al-Khattāb (may Allah be pleased with him), and he said: These are two days that the Messenger of Allah (may Allah's peace and blessings be upon him) forbade fasting: the day of breaking your fast, and the other day in which you eat from your sacrifice

It is prohibited to fast on the days of Eid al-Fitr, Eid al-Ad'ha, and the days of Tashrīq, as they are connected to the day of Eid al-Ad'ha, except for one who does not have a sacrificial animal, as it is permissible for him to fast on the days of Tashrīq.
Ibn Hajar said: it is said: The benefit of describing these two days is to indicate the legal cause for the obligation of breaking the fast therein, which is to distinguish them from fasting and to show the completion of the fast by breaking it afterward. The other cause is the sacrifice that is meant to draw close to Allah by slaughtering it to eat from it.
It is recommended for the speaker to mention in his sermon what relates to the time at hand in terms of rulings and to be mindful of relevant occasions.
Permissibility of eating from the sacrifice.