If a liturgical solemnity falls on a Sunday and is postponed to the day before or after, Catholics are obliged to attend Mass, according to a reply from the Dicastery in September, which was published on the "X" platform (formerly Twitter) on Wednesday.
The background to the Roman response is a so-called "dubium" (request for clarification) from the Bishop of Springfield, Thomas Paprocki, who is also Chairman of the Canon Law Commission of the US Bishops' Conference.
According to media reports, differences of opinion have arisen between the Canon Law Commission and the Worship Commission of the Bishops' Conference regarding the Solemnity of the Immaculate Conception on 8 December.
As reported by the portal "The Pillar", the Divine Service Commission is of the opinion that the obligation for this year has been cancelled due to the postponement.
The background to this is the regulation of the Bishops' Conference, which has been in force since 1992, according to which the obligation to attend mass is cancelled if certain high feasts are postponed. Paprocki's commission was of a different opinion and had asked Rome for clarification.
Mandatory day - despite postponement
"Canon law does indeed determine which feast days are to be celebrated as holy days," reads the letter signed by the Prefect of the Dicastery, Archbishop Filippo Iannone. The Code of Canon Law (CIC) therefore does not provide for any exceptions.
"For this reason, these feasts are always holy days, even if the aforementioned postponement occurs," the prefect continued. "Therefore, this year, the day to which the Solemnity has been postponed must be celebrated as a feast day."
However, the letter from the dicastery refers to the legal principle that "no one is bound by the impossible" and that if "a serious reason makes it impossible for a person to attend Holy Mass, no dispensation is required".
Behind the feast of the Conception of the Virgin Mary is the conviction that Mary is free from original sin. Pope Pius IX (1846-78) elevated this doctrine to infallible dogma on 8 December 1854.
The feast has its origins in the Byzantine Church of the East, where a feast of the "Conception of St Anne" was established around 700 AD. St Anne is the mother of Mary. The feast came to France and England via Italy.
There the emphasis shifted to Mary, whose immaculate conception was now emphasised. Pope Sixtus IV (1471-84) officially introduced the feast for these regions in 1476. Clement XI (1700-21) extended it to the entire Roman liturgy in 1708.
In Austria, Italy and some other countries, the Solemnity of the Assumption is a public holiday - unlike in Germany.
On this day, the popes traditionally go to pray at the Marian Column on the Spanish Steps in Rome.