For Jews, Friday night begins the holiest day of the year, Yom Kippur. The idea behind the holiday is that it is our final chance to make teshuva, repentance, for our sins. These can be sins against ourselves, against others, or against God. The phrase that denotes this most is, “On Rosh Hashana [(the Jewish new year)] it is written, and on Yom Kippur it is sealed.” The parable here is that God is reviewing our actions and decisions from last year, writes our attitude and fate into the “book of life,’ and seals it on Yom Kippur.
To highlight the solemnity of this day, Jews often fast from sundown to sundown. We don’t even drink water. So, unlike Rosh Hashana, this is not really a joyous holiday. As Time magazine explains in this video, therefore, it doesn’t make sense to wish someone a “happy Yom Kippur.” Instead wish someone an easy fast, or simply a good yuntif (holy day).