This phrase is used here to mark a break in the main story line. Here Luke starts to tell a new part of the story. AT: "Now when the baby was eight days old" or "Then, on the eighth day after the baby was born" (See: [[rc://en/ta/man/translate/writing-newevent]])
Here "eighth day" refers to the time after the birth of the baby. AT: "when the baby was eight days old" or "on the eighth day after the baby was born" (See: [[rc://en/ta/man/translate/translate-ordinal]])
This was often a ceremony where one person circumcised the baby and friends were there to celebrate with the family. AT: "they came for the baby's circumcision ceremony" (See: [[rc://en/ta/man/translate/figs-explicit]])