Ninth of Thermidor...
When: July 27, 1794
What: Robespierre guillotined
After the French Revolution, the French calendar was remade. The 7-day week changed to 10 days, the months and days were renamed and the calendar began when the republic began.
After the Great Terror, the last remaining leader of the French Revolution, Robespierre, was guillotined on the 9th of the month of Thermidor, the second year of the republic (which translates to July 27, 1794). Robespierre, after ordering the deaths of many French citizens during the Great Terror, found himself under the guillotine's shadow himself.
I'm not sure if "Happy 9th of Thermidor" is an appropriate phrase here and I doubt this is a sufficient post to give the day credit. However I mixed up the dates and thought it was the 29th, so when I realised it was today, I was a little late in formulating a post. Anyway, if you are curious, there are lots of great websites on the internet talking about the French Revolution!