Mother’s Day in Spanish is El Día de la Madre – or literally, the day of the mother. To say Happy Mother’s Day in Spanish you’d say Feliz Día de la Madre.
To pronounce Happy Mother’s Day in Spanish, say the following, remembering that the capitalized syllables are stressed: feh-LEESS DEE-ah deh lah MAH-dreh
Even among native Spanish speakers, there is some argument about this. You will see greeting cards in Spanish that read El Día de las Madres.
Mother’s Day in English is singular, so a direct translation would use the singular as well. An American woman popularized the holiday and insisted it should be singular, so that it was about a person or family honoring their own mother specifically and not a general honor for all mothers everywhere. Many countries around the world adopted the American occasion and so it makes sense to say Happy Mother’s Day in Spanish as Feliz Día de la Madre.
But either way you choose to say it, your Mother’s Day greetings in Spanish will be understood and appreciated.