Updated speculation... we depart on the Divina on January 6th, 2015!
Cloud 9 (actually) just announced two Holy Ship's for 2015 (both officially sailing on the Divina): January 3rd - 6th, and February 25th to 28th. http://holyship.com/
The current MSC site has the Divina returning from a regular cruise on the 3rd, and then a 14-day gap of open days! After Holy Ship, there are 11 more open days. (... but the MSC site still lists a regular 7-day cruise during the February Holy Ship dates, so they are obviously not fully updated)
If we are in that 11-day window, they will have to either transition to a 4-day Jamcruise (putting the Divina back on her regular departure schedule), or sub in one 6-day cruise to make up the difference. Or... 11-night Jamcruise? Haha!
My guess is JC will leave right after Holy Ship returns, on Tuesday January 6th. There is a bunch of setup that makes it worth their while to do these back-to-back, right? I would love a January 6th departure date, so here's hoping...!