Pesachim, Chapter 6, Mishnah 1. This mishnah deals with those aspects of the Pesach offering that override Shabbos. We know that bringing the Pesach offering is permitted on Shabbos, but certain parts of the process may be done on Shabbos, while others may not.
The mishnah opens: "Eilu devarim baPesach shedochin es haShabbos" - these are the elements of the Pesach offering that override Shabbos when the fourteenth of Nissan falls on Shabbos, the day on which the offering is meant to be brought.
The things that override Shabbos:
"Shechitaso" - the slaughtering of the offering.
"Uzerikas damo" - the sprinkling of its blood on the altar. Sprinkling the blood is not forbidden by Torah law, and at most involves a rabbinic prohibition. In fact, Tosafos explain that the sprinkling of the blood is mentioned here only in connection with the slaughtering, since performing it on Shabbos involves no prohibition whatsoever.
"Umikui keravav" - cleaning out the waste from inside the intestines. If it were left in the intestines until evening, the intestines would spoil; the intestines are edible when they are cleaned out, but if they are left for a long time they become ruined. Therefore they are cleaned on Shabbos. It is explained that this prohibition is rabbinic, because it resembles the melachah of threshing - extracting something from its natural casing and emptying it out of the place where it naturally grows. Even though it is rabbinically forbidden, it was permitted so that the intestines should not spoil.
"Vehekter chalavav" - burning the fats that are offered on the altar. Now, the burning of the fats is valid all night, and it could have been done after Shabbos, but since there is the principle that "a mitzvah is most beloved in its proper time," and since the halachah is that this burning comes as part of the Pesach offering, it is done even on Shabbos.
The things that do not override Shabbos:
"Tzeliyaso" - roasting the Pesach offering.
"Vehadachas keravav" - the additional rinsing of the intestines. Not the actual removal of the waste, but the further rinsing, which is not essential. These do not override Shabbos, and they are to be done after Shabbos.
The mishnah continues and lists three additional items:
"Harkavaso" - carrying the lamb on one's shoulder and bringing it to the Beis HaMikdash.
"Vahava'aso michutz latechum" - bringing it from outside the Shabbos boundary.
"Vachatichas yavalto" - removing a wart that could be considered a blemish and a defect in the offering.
"Ein eilu dochin es haShabbos" - these are things that could have been done before Shabbos, and for that reason they do not override it; Shabbos should not be set aside on their account, since they could have been taken care of beforehand. As we will see in the next mishnah, this is a principle that Rabbi Akiva employs.
Rabbi Eliezer's dissenting view:
Rabbi Eliezer disagrees and says that these things do override Shabbos. All three of them involve nothing more than rabbinic prohibitions: carrying it on one's shoulder within Jerusalem and bringing it from outside the Shabbos boundary are rabbinic, and even cutting off the wart can be done in a manner that is only rabbinic - if it is a dry wart, that is rabbinic, and even if it is moist, it can be cut with a shinui, in an awkward manner, thereby reducing the prohibition to a rabbinic one. Even so, the first Tanna holds that they do not override Shabbos, while Rabbi Eliezer holds that they do - one may desecrate Shabbos for the sake of these things, so that the Pesach offering can be brought.