Edited to correct start date to December 12 throughout
French press is reporting this morning that not only did the four main unions representing rail workers file this morning the strike notice for their planned November 21 strike (as highlighted by JoLui in another thread), they also filed another strike motion for a strike that would start December 12 and be renewable daily for 10 days.
So if this gets really bad, they could strike from December 12 to December 23.
Keep in mind too that a rail strike always starts with slowdowns the evening before the first date of the strike and continuing into the morning after the last date of the strike.
So problems would start on December 11 in the evening (at 7 pm) if they don't get what they want from the government by then.
Keep in mind that filing a strike motion now is a way of getting the government's attention. Maybe they will come to agreement before then, and the unions will call off the strike. But it's a bit unusual for all four main unions to be so unified in calling for the strike.
To keep in mind if you plan on traveling by train in France in December, for example from Paris to Strasbourg for the Christmas markets . . . .
Sorry to be the bearer of bad news.
If the strike goes ahead, the unions are required to inform the SNCF 48 hours ahead of time which workers plan on participating in the strike, and then the SNCF makes its plans based on that for which trains it will be able to run on strike days. The practical effect is that the traveler learns from SNCF via email or text usually the evening before their planned travel if their train will run the next day.