The traditional Meelad is bid’ah, it has no proof. However, keeping fast on Monday is Mustahab (desirable), the Prophet (صلي الله عليه وسلم) used to keep fast on that day. It is in the Hadith:
سئل رسول الله عن صوم الإثنين فقال فيه ولدت وفيه أنزل علي (رواه مسلم)
“Prophet (صلي الله عليه وسلم) was asked about keeping fast on Monday, so he replied: on that very day I was born and the revelation came upon me on the same day”.
It is said in another Hadith: “Good deeds are being presented on Monday and Thursday and I like my good deeds to be presented while I am keeping fast.” (Tirmizi)
These are the virtues and reasons behind keeping fast on Monday. The person whom Allah has given strength should keep fast on that day, it is a matter of great virtue.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband