Jus for your sentence, the correct form is "Est-ce que 'c'est dimanche' est un colloquialisme ?". Also in french, we leave a space between "?" and the end of the phrase. ^^ Oh and, we use more "on est dimanche" than "nous sommes", generally using "nous", if we're not talking about a group, is more formal than "on", but "on" is really used. "On est bien là." "On est content." "On est venu puis reparti."
That's just the way it is. It's not like that sentence in that it involves the name of a day of the week, which brings about a completely new set of rules. (And really, one question mark is enough. I even bet that the probability that you'll get your question answered is inversely proportional to the number of question marks you use.)