Since December 15, 2020, a second curfew from 8 p.m. to 6 a.m. has been set up throughout the French territory to limit the spread of the Covid-19.
Furthermore, as of January 2, 2021, a curfew with adjusted hours, from 6 p.m. to 6 a.m., has been implemented for 15 French departments.
The number of French departments impacted is likely to change the next few days.
Thus, any posted worker whose activities cannot be postponed or done in remote working during these time slots must carry a business trip certificate provided by the employer.
Moreover, the restrictions to enter the French territory are maintained.
Source: French Government (in French)