I also noticed Mnemosyne missing, and wondered about the implications of the Titaness of Memory missing.
However, I couldn’t find any specific implications about the impact it might have on the puzzle.
As celestial objects chosen are all anomalous:
Theia - Theoretical destroyed planet
Themis - Identified in 1905, but turned out to be an optical illusion
Phoebe - Orbits in a different direction/plane, likely a captured object from the Kuiper Belt
Tethys / Dione - Share orbits with smaller objects (Telesto/Calypso and Helene/Polydeuces respectively)
I took this as a hint that these holidays “disrupt” the calendar:
I’ve submitted a memory attempting to describe this pattern and reasoning.
However, 2016 could be a nod towards leap years and extra days being added into the calendar.