My faather (may Allah سبحانه وتعالى be pleased with him) said in his treatise to me: "There is no harm in paying Zakat al-Fitr from the first day of the Month of Ramadan until its last day. It remains Zakat until the Eid prayer is performed. If it is given after the prayer, it is considered a general charity. The best time to pay it is on the last day of Month of Ramadan."
Commentary #5