This is the Solemnity of the Assumption of the Blessed Virgin Mary. That Feast is the traditional day for the celebration of First Vows, at the conclusion of Novitiate in the Society of Jesus.
The day calls to mind the vows of Saint Ignatius and his first companions. They professed vows in the Montmontre section of Paris on August 15, 1534.
Jesuits profess First Vows after the two-year Novitiate. It is only later, after the period of Tertianship, that we take Final Vows. I think that August 15 (at least in this part of the world) is the standard date for novices to pronounce First Vows.