The Jubilees of 1390 and 1423
The celebration next following was held in 1390, and in virtue of an ordinance of Pope Urban VI, it was proposed to hold a Jubilee every thirty-three years as representing the period of the sojourn of Christ upon earth and also the average span of human life. In 1400, so many people came to Rome, that Pope Boniface IX granted the indulgence again, even though he had not decreed a Jubilee year previously.
Another Jubilee was proclaimed by Pope Martin V in 1423 (33 years after the last proclaimed Jubilee in 1390), but Pope Nicholas V, in 1450, reverted to the quinquagesimal period, while Pope Paul II decreed that the Jubilee should be celebrated every twenty-five years, and this has been the normal rule ever since.
Read more about this topic: Jubilee (Christianity)