"'Lasst uns froh und munter sein' ('Let us be happy and cheerful') is a traditional German Christmas carol from the Hunsrück/Taunus region. [...] The song is traditionally sung on Nicholas Eve (Nikolausabend) on December 5, the evening before the feast of Saint Nicholas of Myra, December 6." (https://en.wikipedia.org/wiki/Lasst_uns_froh_und_munter_sein)