Interestingly, the person who has revealed the most is the Pope himself. On board his papal flight from Brazil to Rome, he told journalists that the canonization date will most likely be on Sunday April 27th
, which marks the feast day of Divine Mercy. Ample time is also needed to work with local authorities, to they can plan accordingly for the hundreds of thousands of people who are expected to travel to Rome.
In the case of John XXIII, who convened the Second Vatican Council, Pope Francis waved the requirement of a second miracle, which means he will be declared a saint, despite the fact that he only one official miracle attributed to his intercession.
As far as John Paul II, a French nun who was cured from Parkinson's disease led to his beatification. His upcoming canonization involves the case of a Costa Rican woman who was cured through the intercession of the Polish Pope, after suffering a cerebral aneurism. She was cured on May 1, 2011, the same day John Paul II was beatified.
At first it was believed that the canonization ceremony would take place in the winter of 2013, but Pope Francis said that would be too problematic for Polish pilgrims who have to bear the country's heavy winter to travel to Rome.