Can I fast the day after ‘Īd?

Two things get mixed here: a specific prohibition and a widespread custom.

The prohibition covers the day of ‘Īd al-Fitr, the first of Shawwāl, on which fasting is not permitted. The reason is direct: it is the day appointed for eating and celebrating the end of the month of fasting, and fasting it contradicts that purpose.

The custom of celebrating ‘Īd over three days is real and well established in many countries, but it is social. It does not turn those days into days on which fasting is forbidden.

So from the 2nd of Shawwāl you may:

• Make up days of Ramadan still owed.
• Begin the six voluntary days of Shawwāl, if that is something you want to do.
• Fast voluntarily without more.

The verse that closes the month and orders the celebration: “The month of Ramaḍān is that in which was revealed the Qur’ān, a guidance for the people and clear proofs of guidance and criterion. So whoever sights the crescent of the month, let him fast it; and whoever is ill or on a journey - then an equal number of other days. Allah intends for you ease and does not intend for you hardship and wants for you to complete the period and to glorify Allah for that to which He has guided you; and perhaps you will be grateful.” (Qur’an 2:185)

One qualification worth stating, so as not to leave a wrong impression: on the six voluntary days of Shawwāl the schools hold different views — some early scholars discouraged them for fear they would be taken as part of Ramadan. That you may fast on the 2nd is one thing; that the six days are recommended is another, and there is no unanimity there. If that practice interests you specifically, ask about it separately.

And the other day on which fasting is forbidden, in case the question arises: ‘Īd al-Adha, and the days that follow it.