The Torah requires every male child to be circumcised on the eighth day after he is born. The obligation to perform circumcision on the eighth day is so powerful, that it will be done on Shabbat, even though it involves activities that are ordinarily prohibited. Performing circumcision on Shabbat, however, is only done if that is the eighth day. If a baby could not have a brit milah performed on time for health reasons, we will not do the circumcision on Shabbat. Similarly, if there is some doubt as to whether the baby was born on Shabbat, we will postpone the brit until after Shabbat.
A certain person once came before Rava and asked him, ‘Is it permissible to perform a circumcision on the Sabbath?’ ‘This,’ the other replied, ‘is quite in order.’ After that person went out Rava considered: Is it likely that this man did not know that it was permissible to perform a circumcision on the Sabbath? He thereupon followed him and said to him, ‘Pray tell me all the circumstance of the case.’ ‘I,’ the other told him, ‘heard the child cry late on the Sabbath eve but it was not born until the Sabbath.’ ‘This is a case,’ the first explained to him, ‘of a child who put his head out of the ante-chamber and consequently his circumcision is one that does not take place at the proper time, and on account of a circumcision that does not take place at the proper time the Sabbath may not be desecrated.
The reason that Rava rules that the baby should not be circumcised on Shabbat, but did not say that the brit should take place on Friday is clear from the version of this story that appears in the She’iltot. There it is told that the child’s cry was not heard on Friday afternoon, rather bein ha-shemashot – during the twilight period when it is not clear whether or not Shabbat has begun. Rava’s ruling was that the baby may have been born on Friday and cannot be circumcised on Shabbat (which would not be the eighth day, but the ninth). At the same time, the baby cannot be circumcised on Friday, since the birth may have taken place on Shabbat (which would make Friday the seventh day). Thus there is no choice but to postpone the brit until Sunday.