es un dia mu lindo
It is a special day.
It would be es un dulce dia (with an accent on the I in dia). This is one of the exceptions where the adjective comes before the object.
Como es su/tu dia ? (formal/informal)
Es un dia bonito.
It's a beautiful day.
es un buen dia
Un leprechaun es un duende.
asking how your day went or was. tell him or her
Approximately "It's a beautiful day."
The exact translation would be "Hoy es un dia glorioso" However, that's not the most common phrase in Spanish. Is more usual to say "Hoy es un gran dia"
¿Como es tu dia, guapa? means How's your day going, good-looking?