Yom Kippur is the holiest day of the year in Judaism, and it is a day of fasting, reflection, and repentance. If you want to greet someone who is observing Yom Kippur, it is important to keep in mind that it is not a joyous or festive occasion, so it is not customary to say "Happy Yom Kippur". Here are some appropriate greetings to use:

• "Have an easy fast" is the best greeting to give to someone observing Yom Kippur in English.
• "Good Yuntif" or "Yom Tov" are Yiddish and Hebrew, respectively, for "Have a good holy day" and can be used for those who are not fasting but are observing the holiday.
• "Gmar hatimah tovah" or "Gmar tov" is a more traditional greeting specific to Yom Kippur, which roughly translates to "a good seal".
• "Tzom kal" is a Hebrew greeting that means "easy fast" and is often used in the days preceding the fast or just as its beginning.
• "Wishing you a meaningful Yom Kippur" is another appropriate greeting in English.

Remember that the purpose of Yom Kippur is to reflect on the previous year and ask for forgiveness for any sins committed, so greetings that focus on meaningfulness and reflection are more appropriate than those that are joyous or celebratory.