What Catholic holiday is named after a fungus?

In 1263, in Bolsena, Italy, an event occurred when a
consecrated host appeared to bleed onto a corporal, the cloth upon which the host and
chalice rested on during the Canon of the Mass. This is known as the Corporal of Bolsena
and it is seen as a miracle confirming transubstantiation, or that the bread and wine
become the blood and body of Christ during Mass. A feast was then established by Pope
Urban IV, who lived during the time of this event. It was the feast of Corpus Christi.
It has since been discovered that a microscopic fungus,  micrococcus
prodigiosus can have spores that germinate in bread dough or meal and give the
appearance of clotted blood. Thus, the wafer in mass may have had those fungi spores
contained within, or perhaps a miracle did occur.