I had a dig in the archive for 2012, sorry about any formatting issues from the cut & paste. "Minus 1" from the dates to have a guess for 2013's one e.g. Saturday 1 December 2012 -> Saturday 30 November 2013.
Taken from:
http://web.archive.org/web/20121016033205/http://www.nationalrail.co.uk/times_fares/groupsave_calendar.html*** 2012 ***
Saturday 1 December First Great Western All services to Bath Spa before 14:00 Bath Chritmas Market
Saturday 1 December First Great Western Services to Newbury or Newbury Racecourse before 13:40 Horse Racing
Saturday 1 December First Great Western All services to Cardiff Central before 14:00 Rugby
Friday 7 December First Great Western Not valid on 14:06, 15:06, 16:06, 17:03, 18:03, 19:03 London Paddington - Penzance.
Overcrowding
Saturday 8 December First Great Western All services to Bath Spa before 14:00 Bath Chritmas Market
Friday 14 December First Great Western Not valid on 14:06, 15:06, 16:06, 17:03, 18:03, 19:03 London Paddington - Penzance.
Overcrowding
Saturday 15 December First Great Western All services to Bath Spa before 14:00 Bath Chritmas Market
Friday 21 December First Great Western Not valid on 14:06, 15:06, 16:06, 17:03, 18:03, 19:03 London Paddington - Penzance.
Overcrowding
Saturday 22 December
First Great Western
All services to Bath Spa before 14:00 Bath Chritmas Market
Friday 28 December First Great Western Not valid on 14:06, 15:06, 16:06, 17:03, 18:03, 19:03 London Paddington - Penzance.