What is the legitimate congratulation for a father on the occasion of his son's circumcision, and has a specific congratulation for this occasion been mentioned in the Sunnah, and is it permissible to hold a family feast for relatives?

No specific congratulatory phrase has been reported from the Prophet (peace be upon him) or the predecessors for the occasion of a child's circumcision. However, it is recommended to congratulate the parent of the newborn with what Al-Hasan Al-Basri taught: "May Allah bless you in what He has bestowed upon you, may you be grateful to the Giver, may he reach his maturity, and may you be granted his righteousness." The one being congratulated should respond with "May Allah bless you and bestow His blessings upon you," or similar phrases.

There is no harm in holding a feast for the circumcision if the intention is to show gratitude to Allah and feed the brethren. It is recommended to attend it, provided it does not involve any forbidden acts.