In Milan, it is traditional to consume the last slice of panettone on San Biagio's Day, February 3rd.
San Biagio (St Blaise) was a an Armenian doctor and bishop who lived in the third century AD. He reputedly saved a young boy from choking on a fish bone by giving him a large piece of bread to swallow which removed the bone, and the mother immediately hailed him as a miracle worker.
Unfortunately for the bishop, the news reached the ears of the Emperor Diocletian, who, apparently out of jealousy, had him flayed and decapitated.
Subsequently, Biagio became the patron saint of throat ailments.
The legend that connects him to panettone is that a woman from Milan once asked the friar Desiderio to bless her panettone before Christmas, and the friar told her to return in a few days. However, the woman did not return until much later on San Biagio's day, by which time the friar had eaten it all. As Desiderio was apologetically bringing the empty box to the woman, he opened it and discovered a panettone twice the size of the original.
As a result of this miracle, it has become traditional in Milan to eat the last slice of panettone on San Biagio’s day to protect against throat ailments.