I sent an email to RS and got the following reply:
Thank you for your email, and for your interest in our Best of Rome tour! We currently have 9 seats available on the December 20th departure. For this particular tour start date, all activities will remain the same, with the exception of the Catacombs. We will not be seeing this as a group, and will replace this with another activity (to be determined).
Because of the Christmas holiday, some of the days on this particular departure will be rearranged. For example, we will visit the Vatican on Day 3 instead of Day 4. We will be visiting The Borghese Gallery on Day 4 instead of Day 6, and we’ll be doing “Neighborhood Rome” on Day 6 instead of Day 3. To view the standard itinerary for this tour, please click here https://www.ricksteves.com/tours/italy/rome-2018. We will send out the modified schedule with the final tour documents, about 30 days prior to departure.
If you have any questions, please let us know. We hope to see you in Rome!
Hope this helps others