Yom Kippur is a solemn and reflective holiday, so it is not appropriate to say "Happy Yom Kippur". Instead, here are some appropriate greetings to share with someone observing Yom Kippur:

• "Gmar Tov": This Hebrew greeting translates directly to "a good end" in English. It means that the coming year will be a good year for the person and that their fate will be sealed for goodness in the coming year.

• "Gmar Chatima Tova": This is the customary greeting on Yom Kippur, which means "May you be sealed in the Book of Life".

• "Have a meaningful fast": This is a common English greeting that honors the tone of the holiday while remaining festive for the ongoing Jewish New Year celebrations.

• "Have a good holy day": This is another English greeting that is appropriate for Yom Kippur.

• "Ah gut gebentsht yohr": This Yiddish phrase means "A good and blessed year" and can be used since Yom Kippur comes so close to Rosh Hashanah.

It is important to note that many people who celebrate Yom Kippur dont appreciate it if you say "Happy Yom Kippur," as it doesnt fit the tone of the somber, reflective holiday.