"It is Thursday."
For this sentence, I think that they are indeed irrelevant - when you're talking about the current day and saying "It is Thursday (today)", there is not really an "it" that you can point to, the way you can with "it is an apple".
Similarly with "it is raining" -- there is no "it" that rains.
I don't think you can add αυτό in such cases in Greek.
(Remember also that the dropdown hints are not sentence-specific.)