'First Day' in the Bible

and they assembled the whole community on the first day of the second month. They recorded their ancestry by their clans and their ancestral houses, counting one by one the names of those 20 years old or more,

The one who presented his offering on the first day was Nahshon son of Amminadab from the tribe of Judah.

On the first day there is to be a sacred assembly; you are not to do any daily work.