It is a sunnah, I said so above as well.
However a specific date was not given to celebrate in the form of Mawlid.
Every Monday and Thursday is recommended for fasting unless Eid ul Fitr is on that day.
But a specific Monday with a specific date for his PBUH birthday? Doesn’t exist. As I said the specific date has dispute amongst the well known scholars even regarding his PBUH exact birthday.
That is because Allah SWT is crystal clear about our dates of celebration and thus pinpointed them with exact dates.
The Mawlid celebration not having that level of specification means it was never celebrated in the past by the salaf of this Ummah and yet another reason it’s an innovation.
@shamaan