"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).
Apologies if this was covered in the unit notes, but if היום means "today," how would one say "the day"? Like, for example, if I wanted to translate the Buddy Holly song That'll Be The Day.
That would also be היום... It's mainly context based, but sometimes can cause some confusion.
today is the day: היום זה היום or היום הוא היום. that will be the day: זה יהיה היום but it sounds wierd because of the doubl meaning of היום.