There is no specific ritual act to be observed on 12 Rabi`ul- Awwal in particular. Also, fasting on that day has no special virtue. The best way to worship Allah, attain His Pleasure and show love and adherence to His Message is to follow the Prophet’s footsteps.
Responding to this issue, Sheikh `Atiyyah Saqr, former head of Al-Azhar Fatwa Committee, states the following:
Observing voluntary fasting is a commendable act of worship. It is not restricted to specific times or occasions. However, there are some days on which fasting is haram (prohibited) such as the days of the `Eids (i.e., 1 Shawwal and 10 Dhul-Hijjah); the days of tashreek (i.e., 11th, 12th, and 13th of Dhul-Hijjah); and the day of doubt (i.e., the last day of Sha`ban), though this latter is controversial among the scholars.
There are also some days on which fasting is makruh (disliked) such as fasting Friday alone or Saturday alone.
Moreover, there are some specific days on which fasting is commendable. These are the days of Muharram, days of the four sacred months (Muharram, Rajab, Dul-Qi`dah and Dhul-Hijjah), the day of `Arafah (9 Dhul-Hijjah), the day of `Ashurah (10 Muharram), Monday and Thursday every week, three days (13, 14 and 15) of every lunar month, the six commendable days of Shawwal, and most days of Sha`ban, as the Prophet (peace and blessings be upon him) used to do so.
That is to say, the Prophet’s birthday on 12 Rabi` Awwal, as people used to celebrate it, is not among the days on which fasting is commendable (nor among the days on which fasting is forbidden). Nevertheless, it is by no means commendable to fast on 12 Rabi` Awwal in celebration of the Prophet’s birthday. This is because, it is not completely agreed that the Prophet (peace and blessings be upon him) was born on that day. Rather, some people say that he (peace and blessings be upon him) was born on 9 Rabi`ul- Awwal, and there are still different views in this regard.
On the other hand, fasting on the Prophet’s birthday is rendered blameworthy if it occurs on a day such as Friday because it is blameworthy to intend to fast on Friday only. It is reported in Al-Bukhari and Muslim that the Prophet (peace and blessings be upon him) forbade fasting on Friday alone; he said: “None of you should fast on Friday unless he fasts a day before or after it.”
This is the ruling of fasting on the Prophet’s birthday every year. However, the Prophet (peace and blessings be upon him) used to fast on Monday weekly. He was so keen on it for two reasons. The first reason is that people’s account of deeds are submitted to Allah Almighty on Mondays and Thursdays. So the Prophet (peace and blessings be upon him) preferred that his fasting on those days be included in his account of deeds when offered before Allah (as reported and verified by At-Tirmidhi).
The second reason for the Prophet’s keenness on fasting on Mondays is that Monday was the day on which the Prophet (peace and blessings be upon him) was born and also the day on which he was sent as a Messenger to the worlds (as reported by Muslim). So the Prophet (peace and blessings be upon him) used to fast on that day to express gratitude to Allah Almighty for giving him life and selecting him as a Prophet.
Hence, if Muslims would like to show gratitude to Allah Almighty for the Prophet’s birth and for sending him to guide them to the straight path, then they can perform any voluntary acts of worship such as prayer, charity, fasting or whatsoever. So, there is no specific time to offer supererogatory acts of worship. However, fasting on Mondays is preferable, following in the Prophet’s footsteps.
In conclusion, there are no special acts of worship to be observed on 12 Rabi`ul- Awwal particularly, and fasting on that day has no extra virtue than fasting on any other day. Generally, the best way to worship Allah, attain His pleasure, and show love and adherence to His Message is to follow the Prophet’s footsteps.
It is reported in Al-Bukhari and Muslim that the Prophet (peace and blessings be upon him) said: “He who does not follow my tradition (in religion), is not from me (not one of my followers).” It is also reported in Al-Bukhari on the authority of Abu Ya`la that the Prophet (peace and blessings be upon him) said: “Whoever loves me should follow my tradition.”