Is a Muslim's saying, "I don't care about the stupid or cursed Eid," in a moment of anger, considered an act of disbelief (kufr) or contempt for the religion, even though he knows he did not intend to insult the religion or its rituals?

Ridicule of religion or its manifest rituals constitutes apostasy (kufr and riddah), and an apostate must be executed; this is the responsibility of the ruler. Eid is one of the rituals of Islam, so whoever insults it has insulted Islam and apostatized. However, before ruling on apostasy, it is necessary to verify the fulfillment of the conditions for declaring someone a disbeliever and the absence of impediments, including lack of intention, such as a slip of the tongue in extreme joy or anger. So, if a person insults Eid without intending to demean the religion, they are not to be ruled as a disbeliever. As for one who insults Eid secretly and does not declare it, they are a disbeliever and an apostate between themselves and Allah Almighty, and in this world, they are to be treated as Muslims, while their punishment in the hereafter will be more severe. Such a person must repent to Allah.