Yom Kippur, or the Day of Atonement is the holiest day of the Jewish people. No work is to be done on this day.
The ten days leading up to Yom Kippur are known as the Ten Days of Repentance or the Days of Awe.
Leviticus 23:29  For whatsoever soul it be that shall not be afflicted in that same day, he shall be cut off from among his people.
30  And whatsoever soul it be that doeth any work in that same day, the same soul will I destroy from among his people.
31  Ye shall do no manner of work: it shall be a statute for ever throughout your generations in all your dwellings.
32  It shall be unto you a sabbath of rest, and ye shall afflict your souls: in the ninth day of the month at even, from even unto even, shall ye celebrate your sabbath.