In some countries and cultures "Friday the 13th" is considered a day of bad luck, a day when everything that can go wrong will go wrong.
The French version has the calendar showing Thursday 18 August.
Some of the publishers may have changed the date because they thought that a Friday 13th was a good way of showing why everything was going wrong for the Thom(p)sons, yet another example of
translators taking liberties, though in this case it is minor and amusing.
Mind you, with that pair, everything seems to go wrong no matter what day it is.
On a similar note, in the previous panel, you have Tintin pacing the street deep in thought following his visit to the head of the oil company. In the French version, a poster on the wall had the words "Rappel de classes", a reminder that school is due to resume. It could be that Herge worked on that particular page around the 18th August when a new school term was about to begin.
In the English translation the poster displays "General Mobilisation", adding to the atmosphere of impending war which is a sub-plot in the book.