Rashi on Numbers 9:2/רש"י על במדבר ט׳:ב׳

1במועדו AT ITS APPOINTED SEASON — even on Sabbath; במועדו AT ITS APPOINTED SEASON (stated again in v. 3) intimates that it must be offered even in a state of uncleanness (i.e. when the whole congregation, or at least its greater part is unclean as a result of having come in contact with a human corpse) (cf. Sifrei Bamidbar 65).