Today, 11th November, is an Independence Day of Poland. It is celebrated to commemorate the anniversary of Poland's assumption of independent statehood in 1918 after 123 years of partition by Russia, Prussia and Austria. During this time Poland didn't exist as a country and disappeard from maps.
Here's a short video presenting a history of this holiday.
And here you can watch a nice video showing scenes from last year's celebrations in Warsaw:)