Black Saturday (samedi noir in French) refers, in France, to the day of the year when road traffic is most dense due to the many departures on holiday.
(Traffic problems are exacerbated by France's extreme centralisation, with Paris being the hub of the entire national highway network.)
The black colour is the qualification with which the French government web site Bison Futé designates a day with extrêmement dense (extremely busy) traffic.
[1] The French newspapers call this day samedi noir after Bison Futé's designation.
Around 1:45 AM, a coach collided into passenger cars near Beaune in dense holiday traffic during rainfall.