It is strictly forbidden to fast on the first day of Eid al-Fitr, and you are not required to make up for this day after invalidating it. The same applies to anyone who begins to fast on any of the prohibited days – the two Eids (Eid al-Fitr and Eid al-Adha) and the three days of Tashreeq. However, if you vowed to fast it, it is valid, and you must break your fast and then make it up as an obligation. But if you fasted it due to the vow, you are released from the obligation of the vow, despite the prohibition, as stated in Al-Hadiyah Al-‘Alaiyah, p. 174.