Typically when a saint is recognized in the Catholic Church, their feast day is assigned to the day of their death. This day would technically be the anniversary of their entrance into Heaven.
However, this doesn't always happen, as saints often die on the same day as other saints in the history of the Church.
St. John Paul II, for example, died on April 2, which is normally during Lent or Holy Week. Since his feast day would always be superseded by a different celebration, the Church assigned his feast day to the anniversary of his inauguration Mass on October 22.
Similarly, Bl. John Paul I died on September 28, which is already marked as a day to honor St. Wenceslaus and Sts. Laurence Ruiz and Companions. As a result, his feast day was transferred to the date of his election to the papacy, August 26.
August 26 is also the annual feast of Our Lady of Częstochowa, arguably the biggest feast for Polish Catholics around the world.
Interestingly, Pope John Paul I’s immediate successor, Karol Wojtyła, would be from Poland and would adopt his predecessor' name, John Paul.
St. Wenceslaus, on whose feast day John Paul I died, is a beloved patron of Slavic nations such as Poland.
Moving Bl. John Paul I's feast day to August 26 allows the Church to honor his memory, as well as recalling the prophetic way he prepared the way for his Polish successor.