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Why is May 25 the feast day of St. Gregory VII?

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Gregory VII excommunicating Henry IV — St. Gregory VII Pope Gregory VII, after his expulsion from Rome, laying a ban of excommunication on the clergy “together with the raging king” (Henry IV of Germany), drawing from the 12th-century chronicle of Otto of Freising; in the library of the University of Jena, Germany.

May 25 is the feast day of St. Gregory VII because it marks the anniversary of his death.1

In the tradition of the Roman Catholic Church, the commemoration of saints—often referred to as their natale, or "birthday" into heaven—is historically observed on the anniversary of their passing. St. Gregory VII died on May 25, 1085, in Salerno, which established this date as his liturgical feast day.21