good news of Sayyiduna Isa عَـلَيْـهِ الـسَّـلَام and the dream of my mother that she saw at the time of my birth; at the time of my birth, for my mother, a Noor [i.e. light] appeared which illuminated the palaces of Syria before her.

(Masabata bis-Sunnah, pp. 137)

صَلُّوۡا عَلَى الۡحَبِيۡب           صَلَّى اللّٰهُ تَعَالٰى عَلٰى مُحَمَّد

The Blessed Sahabah also celebrated the birthday

Dear Islamic brothers! The way Allah عَزَّوَجَلَّ has mentioned the advent and the excellence of the Beloved Rasool صَلَّى اللهُ تَعَالٰى عَلَيْهِ وَاٰلِهٖ وَسَلَّم in the Holy Quran and then the Beloved Rasool صَلَّى اللهُ تَعَالٰى عَلَيْهِ وَاٰلِهٖ وَسَلَّم himself has mentioned his blessed birth, similarly, the blessed Sahabah would also repeatedly mention the birth of the Beloved Rasool صَلَّى اللهُ تَعَالٰى عَلَيْهِ وَاٰلِهٖ وَسَلَّم in their own gatherings.

Sayyiduna Abu Sa’eed رَضِىَ اللهُ تَعَالٰی عَـنْهُ has narrated that Sayyiduna Mu’awiyah       رَضِىَ اللهُ تَعَالٰی عَـنْهُ has said: Once the Beloved Rasool صَلَّى اللهُ تَعَالٰى عَلَيْهِ وَاٰلِهٖ وَسَلَّم came near the Halqah [i.e. gathering] of Sahabah and asked: ‘What is impelling you to sit here?’ The Sahabah رَضِىَ الـلّٰـهُ تَـعَـالٰی عَـنْـهُم humbly replied: ‘We are sitting here to do the Zikr of Allah عَزَّوَجَلَّ, and giving thanks [to Him] as He has blessed us with the guidance of Islam and did us a great favour by you.’ He صَلَّى اللهُ تَعَالٰى عَلَيْهِ وَاٰلِهٖ وَسَلَّم then said: ‘By Allah عَزَّوَجَلَّ! Has only this thing impelled you to sit here?’ It was humbly replied: ‘By Allah عَزَّوَجَلَّ! Nothing is impelling us to sit here except this.’ The Beloved Rasool صَلَّى اللهُ تَعَالٰى عَلَيْهِ وَاٰلِهٖ وَسَلَّم then said: ‘[Sayyiduna] Jibra’eel [عَـلَيْـهِ الـسَّـلَام] approached me and informed me that Allah عَزَّوَجَلَّ is proud of you before the Angels.’ (Sunan Nasa’ee, pp. 861, Hadees 5436)  

Dear Islamic brothers! After the abovementioned blessed Hadees, Mufti Ahmad Yar Khan رَحْمَةُ اللهِ تَعَالٰی عَلَيْه has stated: It became clear that it is Sunnah [i.e. way] of blessed Sahabah to hold gatherings and sit in the form of Halqahs    to express thanks for Islam and for the blessed advent of the Beloved Rasool صَلَّى اللهُ تَعَالٰى عَلَيْهِ وَاٰلِهٖ وَسَلَّم. This blessed Hadees is the proof of the gathering of blessed Meelad. (Mirat, vol. 3, pp. 321)