"My birthday party is on Saturday."
Translation:Mi fiesta de cumpleaños es el sábado.
its not, i put the same thing but duolingo often looks for a certain way of saying something, it feels more natural to me to say it the way we did
How do you know that you need to turn this sentence around? Why wouldn't it be: Mi cumpleanos fiesta es el sabado.
It's because "cumpleanos" is considered a characteristic of the party. It's a certain type of party, not just an adjective like red or small. Ex: You wouldn't say, "rock banda." It would be "banda de rock," since it's a rock band, a different category from just band.
Surfing round, I don't find any good explanation -- I think it's just the idiom. Rather like we'd say "festival of song," except we'd also say "song festival" and -- looking at google results -- they just don't do a switch like that.
Prepositions are always arbitrary and weird and never translate consistently between languages. Just when you think some little word means "of" or "by" it turns out that it's used for "on" or "to" under other circumstances.
I think when yo have 2 nouns you use "de" : partido de fútbol,... . When there is a noun and an adjective, you just have noun + adj. : examen difícil.
Would 'fiesta cumpleaños' also be possible, because I'm pretty certain I wrote that in an earlier question and it was marked as correct?