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

• Gmar chatima tova: This is the most common greeting on Yom Kippur. In English, it means "May you be sealed in the Book of Life".

• Ketiva vchatima tova: This greeting means "Have a good writing and sealing" in Hebrew.

• Tzom kal: This is Hebrew for "Have an easy fast".

• Have a meaningful Yom Kippur: This is a good option if you want to wish someone observing the holiday a good day without using a specific greeting.

It is also appropriate to wish someone a "Good Yuntif" or "Yom Tov," which are Yiddish and Hebrew, respectively, for "Have a good holy day".