It’s no coincidence that ‘O Oriens’ is sung on the evening of the winter solstice, as darkness falls on the longest night of the year – the time when winter is at its deepest, but the year’s turning-point has come. In the antiphon and in the Old English poem, Christ is figured as the dawn and the returning sun, appearing in the time of greatest darkness, in the depth of the season the Anglo-Saxons called midwinter. In his De temporum ratione, Bede explains the traditional understanding of the relationship between the church year and the equinoxes and solstices:
very many of the Church’s teachers recount… that our Lord was conceived and suffered on the 8th kalends of April [25 March], at the spring equinox, and that he was born at the winter solstice on the 8th kalends of January [25 December]. And again, that the Lord’s blessed precursor and Baptist was conceived at the autumn equinox on the 8th kalends of October [24 September] and born at the summer solstice on the 8th kalends of July [24 June]. To this they add the explanation that it was fitting that the Creator of eternal light should be conceived and born along with the increase of temporal light, and that the herald of penance, who must decrease, should be engendered and born at a time when the light is diminishing.
–Bede, The Reckoning of Time, trans. Faith Wallis (Liverpool, 2004), p. 87.
The medieval church attached profound importance to the solstices and equinoxes as signs of God’s power over time and the created world. What Bede is explaining here is that early in the development of the Christian calendar, four important feasts were fixed to these four key points in the solar year, marking the births and conceptions of Christ and his forerunner John the Baptist. Christ and John were ‘the children of one year’, the Creator of light born in the darkness of midwinter and the herald who must diminish before him at the time when the year begins to wane.