פירוש על במדבר 9:14

וכי יגור אתכם גר ועשה פסח AND IF A STRANGER SHALL SOJOURN AMONG YOU, AND WILL KEEP THE PASSOVER [ACCORDING TO THE ORDINANCE OF THE PASSOVER … SO SHALL HE DO] — One might think that this verse implies that anyone who becomes a proselyte (גר) must keep the Passover offering rite immediately after his circumcision (even though it has not taken place just before Passover), therefore Scripture states, “yo shall have one ordinance, [both for the stranger, and for him that was born in the land]” (which implies that just as the native brings the offering on the fourteenth of Nisan, so the proselyte, too, must bring it only on the fourteenth of Nisan; cf. Rashi on Exodus 12:48). What the first part of the verse implies is, therefore, the following: “and if a proselyte will sojourn among you, and comes to celebrate the Passover rite together with his fellows, then according to the ordinance of the Passover and according to the manner thereof shall he do” (i.e. ועשה פסח is not a predicative clause — when he becomes a proselyte then shall he offer the Passover, i. e. offer it immediately — but it is a conditional clause, coordinate to וכי יגור אתכם and the apodosis begins with כחקת הפסח etc.) (Sifrei Bamidbar 71).

AND IF A STRANGER SHALL SOJOURN AMONG YOU. The purpose of this is to command the stranger to bring this Passover-offering in the wilderness just as He commanded the Israelites. It is possible that when He said in Seder ‘Bo El Par’oh’ (Go in unto Pharaoh),⁹⁰Exodus 10:1. And when a stranger shall sojourn with thee and will keep the Passover,⁹¹Ibid., 12:48. it refers [only] to the [original] Passover kept in Egypt, for that section was said with reference to the Passover of Egypt, as I have explained there. In that case we might have thought that those strangers who joined us in going out from Egypt, the mixed multitude,⁹²Ibid., Verse 38. should keep the Passover, because they were also included in that miracle [of the exodus], but those who become proselytes afterwards, in the desert or in the Land of Israel, [we might have thought] do not have to bring the Passover-offering, since neither they nor their ancestors were included among [those of whom it is said], and He brought us out from thence;⁹³Deuteronomy 6:23. therefore He had to make them liable here to bring the Passover-offering in [subsequent] generations, in the wilderness and in the Land.

חקה אחת יהיה לכם, here while you are in the desert.