'First Day' in the Bible

However, it was not until the first day of the eleventh month of the fortieth year that Moses addressed the Israelites just as the Lord had instructed him to do.

You have been rebelling against him from the very first day I knew you!

There must not be a scrap of yeast within your land for seven days, nor can any of the meat you sacrifice on the evening of the first day remain until the next morning.