'First Day' in the Bible

And it came to pass in the fortieth year, in the eleventh month, on the first day of the month, that Moses spoke unto the children of Israel, according unto all that the LORD had given him in commandment unto them;

And there shall be no leavened bread seen with you in all your territory seven days; neither shall there any part of the flesh, which you sacrificed the first day at evening, remain all night until the morning.