12th Rabi ul awwal, the date when Prophet peace be upon him and Abu Bakr al Siddique radhiallahanho entered Madina.
Ibn Kathir said:
وكان دخولهما المدينة في اليوم الثاني عشر من ربيع الأول، وهذا أمر معلوم مقرر محرر.
And their entry into Madina was on the 12th of Rabi` al-Awwal, and this is a well-known, established.
(al Bidaya wal Nihaya the year 389 h)
Anas ibn Maalik said, ‘I saw him [the Prophet] the day that he entered upon us [he came to Madeenah] and the day that he died; I have never seen the people so happy like the day when he came, and I have never seen them so sad like the day that he died.’ [Al-Bukhari in At-Taareekh As-Sagheer, and Al-Bayhaqi in Dalaa’il An-Nubuwwah, Taken from Islamweb]
Note: Not a single sahabi said they celebrated birth of Prophet peace be upon him, rather they were happy because He peace be upon him entered Madina on 12th Rabi al awwal by doing Hijrah. And then, no sahabi continued this every year and celebrated seeing Prophet peace be upon him entering in madinah on 12th Rabi al awwal after that.