Because of the 8th and 9th of May, the mayor's office will be closed on the 10th and 11th of May"
This year, the 8th and 9th of May are both holidays. It is common in France for public offices to do long weekends for these holidays, so this office chose to be closed on the 10th and 11th.
This sign is nothing out of the ordinary, but the wording does make it a bit funny.