More details, including other Stagecoach services in Cambridgeshire can be viewed here.
Whippet's timetable alterations are:
25/26 Dec 1Jan - No service
27 Dec amended Saturday timetable
28 Dec Saturday timetable
29 Sunday service
30/31 amended Saturday timetable
See Whippet website here. However, there doesn't seem to be any detailed information about what the 'amended' Saturday timetable is for 27, 30 and 31 December.