Responsa על יבמות 126:21

In the chapter “Ha-ba al Yevimto” (Yevamot 63b), it is said about the magi that came to Babylonia that they exhume the dead because there was rejoicing on their festivals. This implies that it is forbidden to rejoice with them on their festival, and we find no distinction in this regard between idolaters and those who are not idolaters. Therefore, even if this lamb would not be considered a sacrifice, the law would forbid dealing with [the gentiles] to join them in the slaughter and eating. Moreover, since they must slaughter their animals facing south, even on non-festival days, one who slaughters on their behalf incurs the penalty of lashes for violating “do not walk in the ways of the gentiles.”