Q: When do I have to pay zakat al-fitr?
A: Zakat al-fitr becomes obligatory at sunset on the night of ʿEid al-Fiṭr, is permissible before it by two days, preferable on the same day before the prayer, disliked for the remainder of the day, impermissible to delay after it, and is obligatory to make up.
Source: Bidayat al-Abid