Ibn Taymiyah writes:
“Those people who celebrate Milaad through the love and respect of the Prophet [May Allah bless Him and grant Him peace], will be rewarded by Allah.
[Iqtidaa’ us-siraat-il-Mustaqeem, page 294]
He also writes that:
If someone celebrates Milaad with the love and respect of Prophet [May Allah bless him and grant Him peace], he will have a “big reward”. He says that in Muslim communities, Milaad-un-Nabi gatherings are only done with the respect and love of the Muslims for the Prophet [May Allah bless him and grant Him peace]
[Iqtidaa’ as-siraat-il-Mustaqeem, page 297, by Hafiz Ibn Taymiyyah]
Ibn Muhammad bin Abdul Wahab Najdi, in his book, writes:
“Thuwaiba, who was the freed slave of Abu Lahab fed Rasulallah Sallalalhu ‘alaihi wa sallam milk. Abu Lahab freed Suwaiba at the time when she informed him that a son has been born at your brother’s house. After the death of Abu Lahab he was seen in a dream, in which he said ‘I am in severe punishment but this is lessened on Mondays, he showed his forefinger, and said that he would suck from it. This is so because it was with this finger that I freed Suwaiba when she informed of the birth of the Prophet, and she also fed the Prophet Sallalalhu ‘alaihi wa sallam milk”. Ibn Jawzi states: Abu Lahab is that kaafir who has been specially referred to, in the Qur’an. If such a person can be rewarded for celebrating the Milaad of the Prophet Sallalalhu ‘alaihi wa Sallam, then imagine how great the reward would be for a Muslim when he celebrates it.
[Mukhtassar Sirat-ur-rasool, “Milaad-un-nabi”, by Ibn Muhammad bin Abdul wahab Najdi].
The above is evidence that celebrating the birthday of the Prophet [May Allah bless Him and grant Him peace] in the form of a gathering is permissible. Otherwise Hafiz Ibn Kathir would not have referred to it as a good deed and Hafidhh Ibn Taymiyyah would not have said that there would be a “big reward” for doing so.
Some people even argue that the Holy Prophet was not even born on the 12th Rabbi Awwal. This argument is baseless, as Hafiz Ibn Kathir has discussed this in Sirat-un-nabi. He states that the majority of the scholars agree that the Prophet was born on the 12th of Rabi ul-Aawal.
The Ahl as-Sunnah celebrate the Milaad of the Prophet [May Allah bless Him and grant Him peace] with love and respect. If someone does not celebrate it, we do not call them a kaafir because the matter of Milaad does not affect the ‘Aqeedah of a person. The celebrating of this should not cause any divisions among Muslims.
Hafiz Ibn Kathir writes in Sirat un Nabi: “On the day of the birth of Prophet [May Allah bless Him and grant Him peace], Shaytaan cried a lot.”
From Ibn Kathirs narration we can see that to feel unhappy on the Birthday of the Prophet [May Allah bless him and grant him peace] is the way of shaytaan not the way of a Muslim!