Where is Catherine Ségurane Day celebrated every year on November 25th?

In the French city of Nice.

Catherine Ségurane played an important role during the Siege of Nice in 1543 when French and Turkish armies joined forces tried to take the then Savoyan city. According to the legend, Catherine Ségurane, a young laundress, became a heroine right in the moment when the Turks were about to hiss their flag:

When Cathy saw that crescent-mooned flag going up over her city, she went wild with rage. She let out a scream that was heard over the cannon fire and she ran straight for that Turk with her laundry paddle. She whacked him in the head and grabbed the flag as he went flying off the tower. She broke the flag pole over her knee then she ripped the fabric to pieces.

Next she did something that was decidedly unladylike. She turned her back to the soldiers below, bent over, lifted up her skirt and showed them a different moon than the one on their flag. Then she took their ripped up flag and wiped her bottom with it before flinging it down on them. The Turks were horrified by this crazy woman and her desecration of their flag - and they ran. Cathy had saved the day!

The existence of Catherine, however, has never been proven and her heroic act is most probably a legend.

https://curiousrambler.com/nicoise-laundress-saves-the-day/