The 90th mitzvah is that we are commanded to burn sacrifices which have become impure.
The source of this commandment is G‑d's statement (exalted be He), "Any [sacrificial] meat which comes in contact with something impure may not be eaten; it must be burned in fire."
When explaining the reason why on a holiday one may not burn oil of terumah which became impure, the Talmud in Shabbos says, "Refraining from work [on a holiday] is a positive commandment, giving the holiday both a positive commandment and a prohibition. A positive commandment alone cannot push aside both a positive commandment and a prohibition."
The explanation of this is as follows: Doing work on a holiday is prohibited, and if one does work, he transgresses a positive commandment, because he violated the positive commandment regarding the holiday, "It shall be for you a day of rest." He also transgresses a prohibition, namely, "No work may be done on these [days]," i.e., the holidays.
Burning holy things that became impure, however, is only a positive commandment. Therefore, because of the principle just mentioned, that "a positive commandment alone cannot push aside both a positive commandment and a prohibition," one may not burn them on a holiday.
Another statement there indicating the same point is, "Just as it is a commandment to burn sacrifices that have become impure, so too it is a commandment to burn terumah that has become impure."
The details of this mitzvah have been explained in tractate Pesachim and at the end of Temurah.
