According to the website of Brussels Airport a national strike will start at 10.00 Sunday on December 14th and it will for sure last until 10.00 on December 15th, Monday. This is part of a strike amongst the Belgocontrol, Belgian Air Traffic Control, which will cause all air traffic in Belgium to stop, meaning that no flights will land in any of the airports in Belgium and no flights will depart.
You are therefore kindly asked to contact your airline if you are supposed to depart from Brussels or any other airport in Belgium in this period to find out what to do, and most airports kindly ask you not to travel to the airport, because they will not be able to help you any further there.
Be aware of the fact that there are not always the same messages written about the strike online, because on the website of Brussels Airport they claim that the strike starts at 10.00 on December 14th and will last until 10.00 on December 15th, while Charleroi Airport claims that the strike will only cause problems on December 15th.
Make sure to contact your airline in case you are affected!
December 14th, 14:45 update
Based on timetables on the Internet sites of both Brussels Airport and Charleroi it seems as if all flights departing and arriving before midnight on December 14th will operate as normal, but all flight traffic on December 15th will be cancelled. In other words, the flights on December 14th will go as normal, and not as written by Brussels Airport earlier that all flights from 10.00 on December 14th will be cancelled, but only flights arriving or departing on December 15th will be cancelled!