Typically saints are celebrated each year on the day of their death, marking their first day in heavenly bliss.
However, this doesn't always work in the liturgical calendar, and sometimes historians don't know the exact date of a saint's death, or confuse dates on the calendar.
In the case of St. Francis of Assisi, he died in the evening of October 3, 1226. Members of the Franciscan Order continue to recollect his last hours with a service called the "Transitus of St. Francis." The Latin word transitus simply means "passing over," and so it marks the passing over of St. Francis of Assisi into eternal life.
One of the reasons why St. Francis of Assisi was assigned October 4 in the liturgical calendar was because there was some confusion about when he died.
Early on, there were many writings about St. Francis, most of them legendary in character. The earliest biography of St. Francis by the friar-poet Thomas of Celano stated that "on Sunday the fourth day of October, in the city of Assisi ... came forth from the prison of the flesh and took his happy flight to the abode of the heavenly spirits, perfecting what he had begun."
When St. Francis of Assisi was canonized by Pope Gregory IX, the date of his feast was fixed on October 4, " We decree that his birth be celebrated worthily and solemnly by the universal Church on the fourth of October, the day on which he entered the kingdom of heaven, freed from the prison of the flesh."
St. Bonaventure also wrote a Life of St. Francis of Assisi and in his biography, he writes, "Now the holy Father departed from the shipwreck of this world in the year 1226 of the Lord’s Incarnation, on the fourth day of October, at late even of a Saturday, and on the Sunday he was buried."
What makes things confusing is that when you look at a calendar of 1226, October 3 is a Saturday and October 4 is a Sunday. If St. Francis died on Saturday evening, then he technically died on October 3.
Furthermore, in some places in medieval Europe, days ended at sundown, so while St. Francis may have died on the evening of October 3, in their reckoning, he died on October 4.
While October 4 may not be the exact date of St. Francis' death, the Church has decided to continue the long tradition of commemorating his remarkable life on this day.
