It is related:
One day Abu Musa al-Ash`ari, the governor of Basra at the time, wrote to `Umar complaining that the ordinances, instructions, and letters from the Caliph were undated and therefore gave rise to problems linked to the sequence of their implementation. Because of this and other similar problems of undatedness, `Umar convened an assembly of scholars and advisors to consider the question of calendar reforms. The deliberations of this assembly resulted in the combined opinion that Muslims should have a calendar of their own. The point that was next considered was from when should the new Muslim calendar era begin. Some suggested that the era should begin from the birth of the Prophet while others suggested that it should begin from the time of his death. `Ali suggested that the era should begin from the date the Muslims migrated from Mecca to Madina, and this was agreed upon. The next question considered was the month from which the new era should start. Some suggested that it should start from the month of Rabi` al-Awwal, some from Rajab, others from Ramadan, others from Dhu al-Hijja. `Uthman suggested that the new era should start from the month of Muharram because that was the first month in the Arabic calendar of that time. This was agreed upon. Since the Migration had taken place in the month of Rabi` al-Awwal, two months and eight days after the first of Muharram that year, the date was pushed back by two months and eight days, and the new Hijri calendar began with the first day of Muharram in the year of the Migration rather than from the actual date of the Migration.1,
This citation in itself is a strong proof that Mawlid on the very day of birth of the Prophet (s) is something that cannot be rejected for even the Sahaba considered starting the Islamic calendar with this day. While they did not do so, agreeing instead to start with the first of Hijrah, the fact they considered it is proof that its commemoration is valid on the specific day, for in that case it would have become New Year’s day, commemorated annually, as well as an integral part of every believer’s daily life in that it would have been the basis of the calendar and all dates before and after.
1
Narrated by Abu Khaythama in his Tarikh as cited by Ibn Hajar in Fath
al-Bari (Manaqib, min ayn arrakhu al-Tarikh).
