'First Day' in the Bible

Now in the fortieth year, on the first day of the eleventh month, Moses gave to the children of Israel all the orders which the Lord had given him for them;

For seven days let no leaven be used through all your land; and nothing of the flesh which is put to death in the evening of the first day is to be kept through the night till morning.

Let seven weeks be numbered from the first day when the grain is cut.