Bismillahir Rehmanir RaheemAllah Subhanahu wa Ta'ala says in the Holy Qur'an: “Say: ‘In the bounty of Allah Subhanahu wa Ta'ala, and his mercy; Therein let them rejoice.’ That is better then what they Amass.” (10:58)
In this verse, Allah Subhanahu wa Ta'ala tells us that we should be happy when we receive blessings and mercy from him. Without doubt, the Prophet ( Salla Allahu ta'ala 'alayhi wa Sallam) is the greatest mercy and blessing of Allah Almighty. Allah Subhanahu wa Ta'ala says in the Qur'an: “And we sent not to you but a mercy for all the worlds.” (21:107)
“O Prophet Sallallaho Alahi Wa Sallam! The communicator of unseen news. We have sent you as a present beholder and bearer of glad tidings and a warner. And an inviter towards Allah by His command and a brightening sun. And give glad tidings to the believers that for them is great bounty of Allah Subhanahu wa Ta'ala.” (33: 44-47)
“…remind them of the day of Allah…” (14:5)
Hafidhh Ibn Kathir and Qadi Shawkani write that “the day of Allah Subhanahu wa Ta'ala” refers to the day on which Allah Subhanahu wa Ta'alahas done a favour on mankind. For example, the freedom of Bani Isra'il from the slavery of Fir'awn.
Allah Subhanahu wa Ta'ala's greatest favour on mankind was the birth of the Prophet (Sallallaho Alaihi Wa Sallam). This means that the Milaad of the Prophet (Sallallaho Alaihi Wa Sallam) is a “day of Allah Subhanahu wa Ta'ala,” therefore we should remember it.
Hafidh Ibn Rajab writes that “It is recommended to fast on those days on which Allah Subhanahu wa Ta'alahas sent blessings on us. The greatest favor which Allah Subhanahu wa Ta'ala has bestowed us with, is the sending down of the Prophet (Sallallaho Alaihi Wa Sallam). Allah Subhanahu wa Ta'alasays in the Qur'an: ‘I have favored among nations and sent to you a Prophet (Sallallaho Alaihi Wa Sallam)from among you.’”