any of four groups of three days (always Wednesday, Friday, and Saturday) of prayer and fasting, the groups occurring after Pentecost, after the first Sunday of Lent, after the feast of St Lucy (Dec 13), and after the feast of the Holy Cross (Sept 14)
Word origin
Old English ymbrendǣg, from ymbren, perhaps from ymbryne a (recurring) period, from ymb around + ryne a course + dǣg day