All praise be to Allah! May peace and blessings be upon Allah's Messenger, his family, Companions and those who follow his guidance!
There is a repeated question about the ruling on celebrating the Prophet's Mawlid (Birthday), offering special night Prayer, sending peace and blessings upon him, and other practices done by people in this event.
The answer is that it is not permissible to celebrate the Mawlid of the Prophet (peace be upon him) or anyone else, as such a practice is a Bid`ah (innovation in religion) in religion. The Prophet (peace be upon him) did not celebrate such an event and neither did the righteous caliphs, the Companions or those who followed them in the best times of Islamic history.
Whoever introduces into this affair of ours something that we have not commanded, it is to be rejected
You must follow my Sunnah and that of the Rightly-Guided Caliphs. Stick to them and hold fast to them. Avoid novelties, for every novelty is an innovation, and every innovation is an error
And whatsoever the Messenger (Muhammad ﺻﻠﻰ ﺍﷲ ﻋﻠﻴﻪ ﻭﺳﻠﻢ) gives you, take it; and whatsoever he forbids you, abstain (from it).
Indeed in the Messenger of Allâh (Muhammad ﺻﻠﻰ ﺍﷲ ﻋﻠﻴﻪ ﻭﺳﻠﻢ) you have a good example to follow for him who hopes for (the Meeting with) Allâh and the Last Day, and remembers Allâh much.
This day, I have perfected your religion for you, completed My Favour upon you, and have chosen for you Islâm as your religion.
There are many Qur'anic Ayahs to this effect. It is worth mentioning that celebrating the Prophet's Mawlid can lead people to understanding that Allah (Glorified be He) has not perfected the religion for the Muslim nation and that Allah's Messenger (peace be upon him) did not convey what he should have conveyed to the Muslim nation.
It was the duty of every Prophet whom Allah has sent to guide his followers to what he knew was good for them and warn them against what he knew was bad for them.
And then, the best of the speech is embodied in the Book of Allah, and the best of the guidance is the guidance given by Muhammad. And the most evil affairs are their innovations; and every innovation is an error.