Proof of Meelad/Milad in Quran

And (remember) when Allâh took the Covenant of the Prophets, saying: "Take whatever I gave you from the Book and Hikmah (understanding of the Laws of Allâh), and afterwards there will come to you a Messenger (Muhammad SAW) confirming what is with you; you must, then, believe in him and help him." Allâh said: "Do you agree (to it) and will you take up My Covenant (which I conclude with you)?" They said: "We agree." He said: "Then bear witness; and I am with you among the witnesses (for this)."
Then whoever turns away after this, they are the Fâsiqûn (rebellious: those who turn away from Allâh's Obedience).

Aal e Imran 81-82

Comment
Meelad (Milad)/Mawlid is a celebration of birth and Allah is mentioning of the oath when appearance of Seal of Prophets Syyedna Muhammad (SAWS) was'nt made to this world (Although the first among creations was the noor (light) of Seal of Prophets Syedna Muhammad (SAWS)).History tells us that Prophets were the most pious and noblest person of that nation ,even then Allah is taking an oath from that whenever Allah's beloved Seal of Prophets Syedna Muhammad SAWS comes to this world and He is bestowed with prophecy ,it will be obligatory for the Prophets to believe in Him and testify his Prophethood.

This oath was not from every one just from the prophets and Allah says that who so ever amongst you will go against it will not succeed.
This is he expression of love by Allah fo His beloved Prophet Syedna Muhammad (SAWS).that is why Prophet Moses and Prophet Jesus (May peace be upon them) wished to be among the nation of Syedna Muhammad (SAWS).

This was related to the birth of Syyedna Muhammad (SAWS) and we do itin the honour of Seal of Prophets Syedna Muhammad SAWS