'First Day' in the Bible

Now the one who presented his offering on the first day was Nahshon the son of Amminadab, of the tribe of Judah;

Also in the day of your gladness and in your appointed feasts, and on the first days of your months, you shall blow the trumpets over your burnt offerings, and over the sacrifices of your peace offerings; and they shall be as a reminder of you before your God. I am the Lord your God.”

On the first day shall be a holy convocation; you shall do no laborious work.

‘Now in the seventh month, on the first day of the month, you shall also have a holy convocation; you shall do no laborious work. It will be to you a day for blowing trumpets.

Then Aaron the priest went up to Mount Hor at the command of the Lord, and died there in the fortieth year after the sons of Israel had come from the land of Egypt, on the first day in the fifth month.