Pesachim, Chapter 7, Mishnah 10. This mishnah deals with the parts of the Pesach offering that are not fit to be eaten, and with the question of when they are to be burned.
The items that are burned:
"HaAtzamos" - the bones. It is known that one may not break the bones of the Pesach offering. Therefore, a bone that contains marrow which would be fit to eat cannot be opened to reach the marrow inside, because of the prohibition against breaking the bone, and so by its very definition it is something that is not eaten.
"VehaGidin" - certain types of sinews. The Gemara explains that this refers to the portion of the gid hanasheh whose prohibition is not from the Torah but only rabbinic: it is not fit to be eaten, and nevertheless it is considered part of the Pesach offering.
"VehaNosar" - any part of the meat of the Pesach offering that remains beyond the time during which it may be eaten.
The time of burning:
The mishnah rules: "Yisarfu beshishah asar" - all of these are burned on the sixteenth of Nisan.
And what is the law when the sixteenth of Nisan falls on Shabbos? On this the mishnah says: "Chal shishah asar lihyos beShabbos - yisarfu beshivah asar" - on the seventeenth of Nisan, which is the second day of Chol HaMoed. And the reason for this: "She'einan dochin lo es haShabbos velo es Yom Tov" - the burning of consecrated items does not override either Shabbos or Yom Tov, and therefore one must wait until Chol HaMoed, which is essentially a weekday.
The Gemara offers various sources for this law. One of them: on Yom Tov there is a positive commandment, and alongside it a negative commandment of "you shall do no manner of work." The positive commandment to burn nosar, "and you shall burn it in fire," is not strong enough to override the negative and positive commandments of Yom Tov that come together.
In summary: in this mishnah we learned that the bones, the sinews and the nosar of the Pesach offering are burned on the sixteenth of Nisan; and when the sixteenth falls on Shabbos, their burning is pushed off to the seventeenth, since the burning of consecrated items does not override either Shabbos or Yom Tov.