September 11 is the Catalan “independence” day, it’s the day Catalonia became part of Spain, which they were not happy about. People all over the Placa de Catalunya (Catalonia Plaza) in Barcelona were wearing the Catalonian flag.
Catalonia was basically given to Spain in a giant territory horse trade many centuries ago. There was a period of time when Catalan, the language (which is related to but different from Spanish) was banned, and the Catalonian people were generally not treated well.
This year is a special, because it is the 300th anniversary, and there is a vote for Catalonia to separate from Spain. Many people has shirts with “Ara És L’hora” which I believe means “now is the time”. The rest of Spain is not super stoked about the separation and has promised embargoes, which are really a bad time for everybody involved.