Thermidor was the twelfth month in the French Republican calendar, the second month of summer. It started on 18 August (1795-1799) or 19 August (1800-1805).
The French Republican calendar was in use in the Netherlands between 1795 and 1813. You may come across the French names of the months in governement records in this period, such as civil registration records.

French Republican Calendar, circa 1801 (public domain).