Nisan 1 was on Sunday (refer to 7:13:1:4*), so this was on Shabbos. Why did Efrayim's Korbanos override Shabbos?
Da'as Zekeinim: It is because Yosef guarded Shabbos before the Torah was given - "u'Tevo'ach Tevach v'Hachen 1 " (Bereishis 43:16). "Li Gil'ad... v'Efrayim Ma'oz Roshi..." (Tehilim 60:9) alludes to [Hora'os Sha'ah, i.e. the Korbanos of] Eliyahu on Har ha'Karmel, of Gidon, and of Nasi Efrayim on Shabbos; Yehoshua from Efrayim conquered Yericho on Shabbos, and David sinned to open the door for Ba'alei Teshuvah. 2