The phrase to say ‘Merry Christmas’ in Spanish is Feliz Navidad. This is the most commonly used phrase in Spain and other Spanish-speaking countries, irrespective of age, relationship, etc. You are free to use this phrase anywhere, and you won't be wrong. A longer expression you can use is Feliz Navidad y Próspero Año Nuevo. With this, you are not **only wishing them a merry Christmas but also a happy new year.