Our Savior, the Dayspring from the East,
has visited us from on high,
and we who were in darkness and shadow
have found the truth;
for the Lord is born from the Virgin
(Exaposteilarion, tone 3)
The video features the Romeiko Ensemble, a Byzantine choir, performing hymns for the Feast of the Nativity in 2006 at the Hellenic Library in Athens, Greece. About those Byzantine brims:
The cantors (psaltes) wore wide-brimmed hats (skiadion) or tall “bullet” hats (skaranikon) and dressed in special cloaks (kamision and phelonion) girded with a belt (sfiktourion). This cantors’ costume tradition was lost after the Fall of Constantinople in 1453 leaving the cantor dressed only with a black robe (rason) of the Eastern Church. However, for the first time since the Fall, Yorgos Bilalis has joined forces with costume designer Fatima Lavor-Peters to recreate these Byzantine vestments as they are described in several treatises or depicted on Byzantine frescoes and manuscript miniatures.
More on worship in the early Church here on the Liturgica.com site.