Parashas Naso · Passuk 7:12
The one who presented his offering on the first day was Nahshon son of Amminadab of the tribe of Judah.
Chabad.org — English translation + Rashi
Source: Bamidbar Rabbah 13:9.
English: “On the first day” — what does “on the first day” teach? That it was the first of all the days of the week.
In context: The midrash notes that the dedication began on a Sunday, the first day of the week.
Custom
Chazal identify the first day of the princes’ offerings as the first day of the week.