Shabbos, Mishnah 5. This Mishnah addresses the question of how many days after a baby's birth may pass before the mitzvah of circumcision is performed, and this is due to two factors: doubt as to which day counts as the eighth day, and Shabbos or Yom Tov falling near the day on which the circumcision would ordinarily have taken place.
The words of the Mishnah: "Katan nimol lishmonah, letishah, la'asarah, ul'achad asar, lishneim asar, lo pachos v'lo yoser" - a baby is circumcised eight days after his birth, and at times nine days, ten, eleven and even twelve days after his birth. Never fewer than eight days and never more than twelve.
Two rules underlie the calculation:
A circumcision performed before the eighth day does not fulfill the mitzvah of circumcision at all, and therefore one may not circumcise on a day about which there is a doubt that it may be the seventh day.
Only a circumcision that is certainly at its proper time overrides Shabbos and Yom Tov; a circumcision that is not at its proper time, as well as one about which there is doubt whether it is at its proper time, does not override them.
"Heikeitzad?" - the cases in detail:
On the eighth day - "kedarko": in the ordinary situation the baby is circumcised eight days after his birth. If he was born on Tuesday, he is circumcised on the following Tuesday.
On the ninth day - "nolad bein hashemashos": when the baby was born at twilight, regarding which halachah does not decide whether it is day or night, a doubt arises whether he is a Tuesday baby or a Wednesday baby. One may not circumcise on Tuesday, lest it be only the seventh day, and therefore one must be stringent and circumcise on Wednesday, which may already be the ninth day.
On the tenth day - born at twilight on Erev Shabbos: one may not circumcise on the coming Friday, lest it be the seventh day; and one may not circumcise on Shabbos, since it may be the ninth day, and a circumcision that is not certainly at its proper time does not override Shabbos. Therefore he is circumcised on Sunday, which is the first day on which the obligation of circumcision has certainly already taken effect and on which there is no concern of desecrating Shabbos, even though it is certainly not the eighth day but the tenth.
On the eleventh day - Yom Tov after Shabbos: the baby was born at twilight on Erev Shabbos, and Sunday is Yom Tov. Here it is known with certainty that Sunday is not the day of circumcision, since the obligation of circumcision took effect either on Friday or on Shabbos, and one may not desecrate Yom Tov for a circumcision that is not at its proper time. Therefore he is circumcised on Monday - eleven days from the birth.
On the twelfth day - two days of Yom Tov after Shabbos: for example, the two days of Rosh Hashanah falling on Sunday and Monday, and the baby was born at twilight on Erev Shabbos, with a doubt whether he is a Friday baby or a Shabbos baby. One may not circumcise on Friday, lest it be the seventh day; not on Shabbos, lest it already be the ninth day; and not on Sunday and Monday, which are Yom Tov, since only a circumcision at its proper time overrides Yom Tov. Therefore he is circumcised on Tuesday - twelve days after the birth.
A sick baby:
The Mishnah concludes with a separate halachah: a sick baby, for whom circumcision at the time of his illness involves danger, is not circumcised until he recovers. The Gemara explains that when it was an illness that affected his entire body, one must wait seven days from the time of his recovery from the illness, and only afterward perform the bris milah.