Hafidh ibn-e-Kathir Wrote That : There Was Once A King, Abu Sa’eed Malik Muzaffar - A Man Of Good Deeds. in His Kingdom, Wherever He Saw A Lack Of water, He Would Build A Well. He Also Established Many islamic Study Centers. Every Year, He Would Spend Half A Million Dinars On Hosting A Gathering Of His People To Celebrate The Birthday Of The Prophet. ( May Allah Bless Him And Grant Him Peace )
📕( Tareekh Ibn Kathir, See Story Of King Abu Sa’eed, Volume No. 13 )

Some People Have The Opinion That King Abu Sa’eed introduced The Celebrating Of This
Occasion. This is Not True Because Hafidh ibn Kathir States That in The 5th Century Hijri, A King Named Malik Shah Celebrated it Also With A Gathering

📕( Tareekh ibn-e-Katheer Biography Of Malik Shah )