Leviticus
Chapter 16
29
"And it shall be a statute to you forever that in the seventh month, on the tenth day of the month, you shall afflict yourselves and shall do no work, either the native or the stranger who sojourns among you.
Live Psalm 91's Note
Dec 16
This looks like around September 10th.
http://www.lakesideministries.net/TopicalStudies/Times_Weights_Measures/The_Jewish_Calendar.htm