Question: what do you say to someone who celebrates yom kippur?
Answer: The best greeting to give to someone observing Yom Kippur in English is "have an easy fast." For those who are not fasting, but are observing the Yom Kippur, you can wish them a "Good Yuntif," or "Yom Tov," which are Yiddish and Hebrew, respectively, for "Have a good holy day."