The phrase "Happy Holidays" is generally not used is Latin America. You can say "Felices Fiestas" for "Seasons Greetings" though.
Merry Christmas = Feliz Navidad Happy Holidays = Felices Fiestas
Have a happy holiday! Or have a happy Christmas
If you are referring to one holiday: Happy Holiday If you are referring to more than one, such as Christmas and New Years: Happy Holidays.
no, because you can only say "Have a Happy Holiday, or just "Happy Holidays."
no, because you can only say "Have a Happy Holiday, or just "Happy Holidays."
Bonnes vacances
You can say holiday as fiesta, which also means party.
día festivo
Holiday: Festivo, feriado
feliz
I'm happy too in Spanish is Estoy contento también.
The universal language, smile and wave.