cellio | daf bit: Pesachim 70

On the fourteenth of Nisan (the afternoon before the evening of Pesach) everyone was required to bring a lamb or goat to the temple, where it would be sanctified and slaughtered (and then returned to the person for the meal). According to the mishna one also brought a "chagigah", a festival offering, at the same time, but there are arguments about exceptions. The g'mara reports that the sages say we do not bring this extra offering on Shabbat, but Yehudah ben Durtai and his son disagreed. On what basis? Rab said: because it is written "you shall sacrifice the Pesach offering unto the Lord your God, of the flock and of the herd" -- but the Pesach offering must be either a lamb or a goat, and those come in flocks, not herds. Therefore, "the flock" refers to the Pesach offering and "the herd" refers to the chagigah, which can be (e.g.) a bull. (mishna 69b, g'mara 70b)

Performing a sacrifice involves several categories of labor that would otherwise be forbidden on Shabbat. This is ok for the regular festival offerings (e.g. the one that will be brought the next day, the 15th of Nisan), because they are commanded in the torah and God is allowed to trump the laws of Shabbat. But that reasoning wouldn't necessarily apply to any other offering, like the chagigah. So determining that it is also commanded is important.