Yom Kippur is a solemn and reflective holiday, so it is not customary to greet those observing with "Happy Yom Kippur". Instead, there are several appropriate greetings that you can use to acknowledge the holiday and share well wishes with your friends and colleagues who observe it. Here are some of the most common greetings:

• Gmar chatima tovah: This is the most traditional greeting for Yom Kippur. It means "a good final sealing" and references the belief that a persons fate for the coming year is written on Rosh Hashanah and sealed on Yom Kippur.

• Gmar tov: This is a shortened version of "gmar chatima tovah" and also means "a good final sealing".

• Tzom kal: This Hebrew greeting translates to "an easy fast" and can be used to acknowledge the difficulty of fasting on Yom Kippur.

• Have a meaningful fast: This greeting recognizes the purpose of Yom Kippur as a day of atonement and reflection.

• Have a good holy day: This Yiddish and Hebrew greeting, respectively, can be used to wish someone a good day during the holiday season.

• Shana tova: This Hebrew greeting means "happy new year" and can be used during the entire season.

It is important to note that the best greeting to give depends on the person and their level of observance. If you are unsure which greeting to use, you can always ask the person what they prefer.