One who pays zakat al-fitr after the festival prayer has given voluntary charity, not zakah.
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The Shaykh : And one who pays it after the festival prayer has given voluntary charity, not zakah, based on the hadith of Ibn 'Abbas: Sunan Abi Dawud 1609Hasan (Al-Albani) When is the time for paying it? The best time is before the people go out to pray on the day of the festival, and it is permissible to pay it one or two days before.