According to Catholics, when is Luke 22:8?

The Roman Catholic (RC) point of view is Christ died on Friday the 15th of Nisan.

Jesus died on Friday, the fifteenth day of Nisan. http://www.newadvent.org/cathen/08377a.htm

RC would place Luke 22:7 as taking place Thursday afternoon when the lambs were slain. After sunset, they eat. With the day's change at sunset, the Supper would be the 15th of Nisan. Christ would then be crucified Friday the 15th in its view.

It's true, however, that Pope Benedict XVI appears to come down on the side of John's chronology (see quote from his book in other post answer); that is, he believes Christ died on Friday the 13th of Nisan and Passover proper was to come. For what it is worth, no one in tradition (or scripture) I've heard of believed that Christ died on the 13th.

The Catholic Encyclopaedia, discussing the timing of the Last Supper, refers to Luke 22:7, along with parallels Matthew 26: 17 and Mark 14 12 and says:

From these passages it seems to follow that Jesus and his disciples conformed to the ordinary custom, that the Last Supper took place on the 14th of Nisan, and that the Crucifixion was on the 15th, the great festival of the Jews.

According to this, then, Luke 22: 7 happened on the morning of the 14th Nissan. The Last Supper took place that evening and was the Passover meal.

Because the Jews reckoned a day as beginning at sunset, then the evening of the 14th counted as the 15th, so the Last Supper on Thursday evening and the Crucifixion on Friday daytime were both on the 15th.

However, the article adds, it seems in John's Gospel that Crucifixion took place on the 14th, and even in the other Gospels there are tings happening on the day of the Crucifixion which seem unlikely to have happened on Passover.

There is therefore some uncertainty as to whether the day on which the disciples prepared the Last Supper was the 14th (synoptic) or 13th (John).

Pope Benedict XVI discussed the issue in his book Jesus of Nazareth, an excerpt from which can be seen here. He starts with the Synoptic Gospels and says that according to them the Last Supper was the Passover meal. This of course means that Luke 22:7 was on 14th Nissan.

However he goes on:

This chronology suffers from the problem that Jesus' trial and crucifixion would have taken place on the day of the Passover feast, which that year fell on a Friday. ... But despite all academic arguments, it seems questionable whether the trial before Pilate and the crucifixion would have been permissible and possible on such an important Jewish feast day.

He then goes on to consider John's chronology:

Let us now turn to John's chronology. John goes to great lengths to indicate that the Last Supper was not a Passover meal. On the contrary: the Jewish authorities who led Jesus before Pilate's court avoided entering the praetorium, "so that they might not be defiled, but might eat the Passover" (18:28). The Passover, therefore, began only in the evening, and at the time of the trial the Passover meal had not yet taken place; the trial and crucifixion took place on the day before the Passover, on the "day of preparation", not on the feast day itself. The Passover feast in the year in question accordingly ran from Friday evening until Saturday evening, not from Thursday evening until Friday evening.

In that case the incident in Luke 22 7 would have been on Nissan 13.

The Pope Emeritus (as he now is) then points out John's chronology has Jesus's death at the time the Paschal lambs were being slaughtered, and the theological significance of this has led some to argue John altered the chronology to fit the theology.

He discusses in some detail various theories that have ben suggested to reconcile the Synoptic accounts with that of John. He does not find them entirely satisfactory.

One thing emerges clearly from the entire tradition: essentially, this farewell meal was not the old Passover, but the new one, which Jesus accomplished in this context. Even though the meal that Jesus shared with the Twelve was not a Passover meal according to the ritual prescriptions of Judaism, nevertheless, in retrospect, the inner connection of the whole event with Jesus' death and Resurrection stood out clearly. It was Jesus' Passover. And in this sense he both did and did not celebrate the Passover: the old rituals could not be carried out – when their time came, Jesus had already died. But he had given himself, and thus he had truly celebrated the Passover with them. The old was not abolished; it was simply brought to its full meaning.

On this basis one can understand how it was that very early on, Jesus' Last Supper – which includes not only a prophecy, but a real anticipation of the Cross and Resurrection in the Eucharistic gifts – was regarded as a Passover: as his Passover. And so it was.

It is impossible to say that there is a definite Catholic point of view on the matter. However the idea that the chronology associated with John is the correct one is supported by Pope Benedict, and his focus on the Eucharistic Theology, is also Catholic.

On this basis Luke 22: 7 took place on Nissan 13.