Maybe it’s too early in the morning for my brain, but I really can’t understand how one can go from a Thursday to a Friday by adding 10 days. Or 11.
I guess it’s just by fiat? It was just declared that for that special time, warping ahead 10 days still gave a sequential order to the days of the week? I guess it makes sense; after all if Pope Gregory didn’t do that then the historic weekly rhythm would be lost and the holy day of obligation that is Sunday would actually be a “Wednesday” or the historic “4th day of the week”.
So I guess that’s both how and the reason a ten day leap still “preserved” the historic sequence of the week. By simple fiat. So I guess I answered my own question (unless someone disagrees)
Yup, you pretty much answered your own question. They wanted to preserve the days of the week (can’t goof up the Sabbath day), but needed to alter the numbering to synchronize things properly ... so one Thursday was followed by a Friday numbered 10 days into the future. Small wonder Arthur could never get the hang of Thursdays.
Very simple. You just call the following day a Friday, which it would be anyway.