'First Day' in the Bible

And David said unto Jonathan, "Behold, tomorrow is the first day of the month, and I should sit with the king at meat. But let me go that I may hide myself in the fields unto this day three days at evening.

Then said Jonathan to David, "Tomorrow shall be the first day of the moon. And thou shalt be missed, because the place shall appear empty.

“But what have I done?” David replied to Achish. “From the first day I was with you until today, what have you found against your servant to keep me from going along to fight against the enemies of my lord the king?”