in each group, teams played against each other home-and-away in a round-robin format. the group winners and runners-up advanced to the round of 16, while the third-placed teams entered the 2017–18 uefa europa league round of 32. the matchdays were 12–13 september, 26–27 september, 17–18 october, 31 october – 1 november, 21–22 november, and 5–6 december 2017.