Exodus 12:25

והיה כי תבאן אל הארץ AND IT SHALL COME TO PASS WHEN YE BE COME TO THE LAND — Scripture makes the observance of this service dependent upon their entrance into the land of Palestine (Mekhilta), and they were not under any obligation, when in the wilderness, to keep more than the one Passover which they kept in the second year after the Exodus (cf. Numbers 9:1—5), and that, too, only in consequence of a special divine communication. (Cf. Rashi on Numbers 9:1 and כאשר דבר (תוס׳ קיד׳ ל"ז ע"ב ד"ה הואיל ACCORDING AS HE SPAKE — And where did He say that He would give them that land? In the following verse: (Exodus 6:8) “And I will bring you into the land” (Mekhilta).

וְהָיָ֞ה כִּֽי־תָבֹ֣אוּ אֶל־הָאָ֗רֶץ אֲשֶׁ֨ר יִתֵּ֧ן יְהֹוָ֛ה לָכֶ֖ם כַּאֲשֶׁ֣ר דִּבֵּ֑ר וּשְׁמַרְתֶּ֖ם אֶת־הָעֲבֹדָ֥ה הַזֹּֽאת׃

And it shall come to pass, when you shall come to the land which the Lord will give you, according as he has promised, that you shall keep this service.