Have had a quick look and DFDS Christmas and New Year services are detailed on this link:
http://www.dfdsseaways.co.uk/customer-service/dover-dunkirk/travel-update/
which states as follows:
Christmas Eve: last departure 1200
Christmas Day: no sailings
Boxing Day: first departure 1200
27th -30th: full schedule
New Years Eve: last departure 1600
New Years Day: first departure 0800