From the Eurostar webpage:
National Strike Belgium
08 + 15 December 2014 - No Eurostar services operating to and from Brussels
Due to a National Strike in Belgium on the 8th + 15th December 2014, Eurostar services on the Brussels route will only be able to operate to and from Lille Europe on that day.
Due to the scale of the strike we expect no trains or other public transport to run in Belgium. Eurostar are unable to provide any alternative method of transport between Brussels and Lille and we strongly advise our passengers not to travel on these dates.