The adjective "BUONO" ( it is masculine and singular ) becomes BUON when it is placed before a noun. It isn't important if the noun starts with a consonant or a vowel ( e.g., BUON VIAGGIO, BUON AMICO, BUON FRATELLO, BUON UOMO, BUON PADRE, BUON NATALE, BUON ANNO), but there are some EXCEPTIONS. In fact BUONO doesn't change if the noun starts with - S+ another consonant ( BUONO STUDIOSO ) GN- ( BUONO GNOMO) PS- (BUONO PSICOLOGO) PN (BUONO PNEUMOLOGO) z ( BUONO ZIO) Y- (BUONO YOGURT) X- (BUONO XILOFONO) I hope my explanation can be useful.

David, I'm assuming you are asking which it is and you know that it's used like "Merry Christmas" is. The right form is the first. Buono, meaning good, is truncated to buon when placed before a noun that starts with (most) consonants.