"היום יום ראשון."
Translation:Today is Sunday.
Actually, my quick little foray into the etymology of the word says that the root is ראש, which is pronounced "rosh." So the question would probably be better, why rishon instead of roshon? And the answer probably has something to do with vowel changes when a word transitions from noun to adjective. Something similar to how lead (the metal) is pronounced the same as led (past tense verb) even though it's spelled like lead (present tense verb).