The term Hexameron (Greek: Ἡ Ἑξαήμερος Δημιουργία Hē Hexaēmeros Dēmiourgia) refers either to the genre of theological treatise that describes God's work on the six days of creation or to the six days of creation themselves. Most often these theological works take the form of commentaries on Genesis I. As a genre, hexameral literature was popular in the early church and medieval periods. The word derives its name from the Greek roots hexa-, meaning "six", and hemer-, meaning "day". Saint Basil wrote an early and influential series of homilies around 370 AD which figure as the earliest extant Hexameron. Basil originally performed the work as a series of sermons, and later collected them into a written work which was influential among early church leaders.