Groom Choosing is a long-standing tradition in the Barsburg royal family. As the title implies, groom choosing is an event in which fiancé candidates compete to woo the crown princess of the Barsburg Empire and win her hand in marriage. The final decision is made by the crown princess herself, when she chooses the fiancé candidate she likes the most. Fiancé candidates are young men chosen from the Seven Houses of God. Though it can be considered a serious occasion because it determines who the next Emperor will be, it is also a grand masquerade party to which wealthy and influential families are invited for a night of dancing and entertainment.
In the manga, it is explained that Barsburg empresses and empresses-to-be are cursed with short lifespans (later revealed to be caused by the Raphael Project), thus, they must marry and ascend to the throne early in life. This is why the groom choosing tradition was founded.
The exact date of the beginning of the groom choosing tradition is unknown, but it could have been around the time the Raphael Project began. Whether the tradition continued after Roseamanelle Ouka Barsburg destroyed the Raphael Project is unknown, though it is unlikely, considering Ouka's views on arranged marriage.