From the introduction on YouTube:
Saturday, September 27th, is the Memorial of Saint Vincent de Paul: founder of the Ladies of Charity, the Company of Daughters of Charity of Saint Vincent de Paul (with Saint Louise de Marillac), and the Congregation of the Mission (the Vincentians). In addition, he is the inspiration and patron of the Society of Saint Vincent de Paul, founded by Blessed Frédéric Ozanam, whose feast day was September 7th.
The hymn is the first stanza of Ubi Caritas:
Ubi cáritas et ámor, Déus ibi est. Congregávit nos in únum Chrísti ámor. Exsultémus, et in ípso jucundémur. Timeámus, et amémus Déum vívum. Et ex córde diligámus nos sincéro. Ubi cáritas et ámor, Déus ibi est.
Where charity and love are, God is there. Christ’s love has gathered us into one. Let us rejoice and be pleased in Him. Let us fear, and let us love the living God. And may we love each other with a sincere heart. Where charity and love are, God is there.