Shevuos 1:4-5
Shevuos 1:4
If there was no knowledge of impurity, neither at the outset nor at the end, then the goats offered as korban musaf on the three Festivals and on Rosh Chodesh atone; this is the opinion of Rabbi Yehuda. Rabbi Shimon says that only the goats of the three Festivals atone for this, not those of Rosh Chodesh. The goats offered on Rosh Chodesh, he says, atone for a ritually clean person who ate ritually unclean sacrifices. Rabbi Meir says that all the goats atone the same for impurity vis-à-vis the Temple and sacrifices. Rabbi Shimon said that the Rosh Chodesh goats atone for ritually clean people who ate ritually unclean sacrifices, while the goats of the three Festivals atone for people who had no knowledge of ritual impurity, neither at the start nor at the finish. The goat of Yom Kippur atones for people who had no knowledge of impurity at the start but who subsequently became aware of it. The Sages asked him whether it was permitted to offer a goat designated for one of these day on a different one of these days; he replied that it is. They then then asked how that is possible seeing that, according to Rabbi Shimon, they atone for different things. He replied that they are interchangeable because they all atone for ritual impurity related to the Temple and its sacrifices.
Shevuos 1:5
Rabbi Shimon ben Yehuda said in the name of Rabbi Shimon (bar Yochai) that the Rosh Chodesh goats atone for a ritually clean person who ate ritually unclean sacrifices. The goats of the three Festival, in addition to this, also atone for sins where a person had no knowledge of impurity, neither at the start nor at the finish. The goat of Yom Kippur, in addition to these, also atones for sins where there was no knowledge of ritual impurity at the beginning but there was knowledge at the finish. The Sages asked him whether it was permitted to offer a goat designated for one of these day on a different one of these days; he replied that it is. They then said that it makes sense that the goat for Yom Kippur could be offered on Rosh Chodesh (since it atones for what the Rosh Chodesh goat does), but how can the Rosh Chodesh goat be offered on Yom Kippur to atone for a sin that is not in its purview? He replied that they are interchangeable because they all atone for ritual impurity related to the Temple and its sacrifices.