Pale' Oscar Lujan Calvo, with American officials, Dueñas family members and others, at the original grave of Pale' Dueñas in Ta'i in March of 1945
JULY 12, 1944 is observed as the official and recognized date of death of Father Jesus Baza Dueñas, and the other three killed along with him, including his nephew, one-time Island Attorney Edward Camacho Dueñas.
Some of those observing the July 12th date are :
Governor Carlos G. Camacho's Proclamation 70-24 (1970) making July 12th the official anniversary
Antonio M. Palomo, historian, in many of his writings
Guam Recorder, and other publications
Julius Sullivan, OFM Cap, in The Phoenix Rises, the 1957 church history book
However, July 13 is offered as the day of his execution in an affidavit signed only in June of 1945, only (little less than) a year after the death of Fr Dueñas. This affidavit was signed by Pale' Oscar Calvo, who got his information from the Saipanese interpreter, Joaquin Dueñas, who was present at the beheading in Tå'i.
This, as far as I know, is the only source for the alternative date of July 13.
And yet - curiously enough - the same Pale' Oscar places a bronze plaque above the final resting place of Fr Dueñas in the sanctuary of Inarajan Church and states the date of death as July 12.
Perhaps, because the killings were done in the darkness before the dawn (about 4AM the affidavit states), as the end of an ordeal that began on July 12, people remember the death as occurring on July 12.
Or, perhaps, when he wrote the affidavit, Pale' Oscar got his dates wrong.
As it is, July 12 is the day everyone observes his martyrdom.
The Memorial Plaque in Inarajan Church