Yom Kippur is a solemn and reflective holiday, so it is not customary to greet those observing with "Happy Yom Kippur". Instead, appropriate greetings for Yom Kippur are more about meaningfulness and reflection. Here are some greetings that you can use:

• "Have an easy fast": This is the best greeting to give to someone observing Yom Kippur in English. Fasting is hard, so this greeting is a way to acknowledge the difficulty of the fast and wish them an easier time.

• "Good Yuntif" or "Yom Tov": These are Yiddish and Hebrew, respectively, for "Have a good holy day." They are appropriate for those who are not fasting but are observing the holiday.

• "Gmar hatimah tovah" or "Gmar tov": This is a more traditional greeting specific to Yom Kippur, which roughly translates to "a good seal." It references the belief that the book of life, which determines an individuals fate for the coming year, opens on Rosh Hashanah and is sealed at the end of Yom Kippur following the period of repentance.

• "Tzom kal": This Hebrew greeting translates to "an easy fast." It acknowledges the difficulty of the fast and wishes them an easier time.

• "Have a meaningful fast": This greeting acknowledges the solemnity of the holiday and wishes them a meaningful experience.

Remember that these greetings can be used in English, Hebrew, or Yiddish, depending on your preference and the preference of the person you are greeting.