Parashas Naso
וַיְהִ֗י הַמַּקְרִ֛יב בַּיּ֥וֹם הָרִאשׁ֖וֹן אֶת־קׇרְבָּנ֑וֹ נַחְשׁ֥וֹן בֶּן־עַמִּינָדָ֖ב לְמַטֵּ֥ה יְהוּדָֽה׃
The one who presented his offering on the first day was Nahshon son of Amminadab of the tribe of Judah.
Torah Temimah points to TT (verified).
ביום הראשון — מה ת״ל ביום הראשון? לומר שהיה ראשון לכל ימות השבוע.
English: “On the first day” — what does “on the first day” teach? That it was the first of all the days of the week.
Custom
Chazal identify the first day of the princes’ offerings as the first day of the week.