Yom Kippur is the holiest day of the year in Judaism, and it is a solemn day of fasting, repentance, and self-examination. If you want to greet someone who is observing Yom Kippur, it is important to be respectful and mindful of the solemnity of the day. Here are some appropriate greetings to use:

• "Gmar chatima tova" or "Gmar tov": These Hebrew greetings mean "a good final sealing" and are based on the belief that a persons fate for the coming year is written on Rosh Hashanah and sealed on Yom Kippur.

• "Tzom kal": This Hebrew greeting means "have an easy fast" and is appropriate to use for those who are fasting.

• "Have a meaningful fast": This is a respectful way to acknowledge the day for those who are observing but not necessarily fasting.

It is important to avoid saying "Happy Yom Kippur" as the day is not necessarily joyous and festive, but solemn and reflective.