Is it permissible to fast on the day of the Prophet's birthday, peace and blessings be upon him, and on people's birthdays in general, following the example of the Prophet, peace and blessings be upon him, due to his saying concerning fasting on Monday: "That is the day I was born"?

The Prophet, peace and blessings be upon him, used to fast on Mondays out of gratitude to Allah for the blessing of his birth on that day, and because it is a day when deeds are presented, and he loved for his deeds to be presented while he was fasting. Fasting every Monday is a good deed and in accordance with the Sunnah (definition). However, singling out a specific day of the year to fast it as a celebration of the Mawlid (Prophet's birthday) is an innovation (bid'ah). The celebration by Muslims of what is called "Christmas" (referring to the Prophet's birthday celebration) is an unlawful innovation. The feasts of Muslims are only Eid al-Fitr and Eid al-Adha. The righteous predecessors were not known to celebrate the Mawlid or to fast on a specific day for its sake.