To wish someone a happy Rosh Hashanah, the traditional greeting in Hebrew is "Shanah tovah" which means "Good year." You can also say "Shanah tovah u'metukah," meaning "A good and sweet year," to wish someone a year that is both good and sweet. In English, simply saying "Happy Rosh Hashanah" or "Happy New Year" is also very common and perfectly acceptable. Additional greetings include wishing someone to be inscribed and sealed for a good year with phrases like "Leshanah tovah tikatev v’tichatem." For a more casual or universal holiday greeting, "Chag sameach" meaning "Happy holiday" can be used. Here are some common ways to wish someone a happy Rosh Hashanah:

• Shanah tovah (שנה טובה) — Good year.
• Shanah tovah u’methukah (שנה טובה ומתוקה) — Good and sweet year.
• Happy Rosh Hashanah!
• Happy New Year!
• L’shanah tovah tikatev v’tichatem — May you be inscribed and sealed for a good year [more formal].
• Chag sameach — Happy holiday (more general for Jewish holidays).

Using these phrases with warmth and sincerity is what matters most when wishing someone well during Rosh Hashanah.