Yom Kippur is a solemn day of fasting and reflection for the Jewish community. If you want to greet someone observing Yom Kippur, here are some appropriate greetings to use:

• "Gmar chatima tovah" ("May you be sealed in the Book of Life")
• "Tzom kal" ("Have an easy fast")
• "G'mar tov" ("Good sealing")
• "Chag sameach" ("Happy holiday")
• "Gut yontiff" ("Good holiday")

It's important to note that Yom Kippur is not a happy holiday, so it's best to avoid saying "Happy Yom Kippur". Instead, you can acknowledge the holiday and share well wishes with your friends and colleagues who observe it.