March 17, 1861
Italian Unification Day: His Majesty Vittorio Emanuele II officially became King of the newly formed nation of Italy. Prior to this date there were no Italian citizens because there was no Italy. Consequently, while the research may be interesting for family history reasons, your jure sanguinis application should never require official records too much older than this date.
This date also means that if your only “Italian” ancestor is Julius Caesar, Christopher Columbus, or Leonardo da Vinci then you do not qualify for jure sanguinis citizenship recognition. The earliest Italian ancestor in your chain must have been alive at any moment when Italy was actually a nation. Then-current citizens of most of Italy’s antecedent states — for example, citizens of the Kingdom of the Two Sicilies — living anywhere in the world automatically acquired Italian citizenship on this date.
Italy did not actually enact its first civil code regulating citizenship until 1865, but that code is retroactive to March 17, 1861.