The term describing these words is auxiliary verb.
a verb that adds functional or grammatical meaning to the clause in which it appears—for example, to express tense, aspect, modality, voice, emphasis, etc.
As for the process of turning a sentence into a question, I'm not aware of any specific term for it. I'd just say "forming an interrogative" or "converting into a question" or something similar.
That said, as deadrat and StoneyB rightly point out, what is happening here is called subject-auxiliary inversion; for example, the statement form of your first example would be "It is Thursday today.", while in question form the subject and verb are switched around so the question form begins "is it" instead.