Every year, at the end of the summer Barcelona celebrates its city fest, La Mercè. There are tons of activities, from local traditions, music concerts, fireworks, and exhibitions. The most active days are Sep 22nd to 25th but events start as early as today.
If you happen to be in town I recommend you take a look at the fest webpage where you can filter activities by date, location
I'd like to share four of the most iconic events for foreigners.
1. Gegants i Capgrossos. Giant and bighead figures.
Most Catalan towns have their own giants, and they take them out on a parade the day of the local fest. As Barcelona has different districts which used to be different villages, there are giants from all these parts of the city. They represent ancient kings or lords, as well as famous locals. It is a family event mostly enjoyed by kids. You can also visit La Casa dels Entremesos museum where they are kept during the year.
2. Correfoc - Catalan word meaning the sum of two words together Run + Fire
It is also a tradition in many villages in Catalonia. Big dragon figures are recreated and "driven" by devils who play with noisy fireworks chasing the audience. If you plan to attend, I encourage you to wear cotton, and a hat, and cover all your body as there are sparks all over. It is fun, but I would not recommend being very close to the parade if you plan to attend with kids
3. Human Castles
These human figures are starting to be known all over the world, there is a very famous contest in Tarragona every other year and there are some small towns which have the highest towers. It is very impressive to watch how the towers are built next to them. For higher ones, they even encourage the audience to help.
Here's a documentary National Geographic created a few years ago.
4. Piromusical event - Fireworks, music, water, and light.
On the closing day, there is a unique fireworks event, which is held in the old exhibition area of the city, where the magic fountain is located. The fountain was built in one of the old Barcelona Universal Exhibitions and was completely reconstructed before the 1992 Olympic games. You may recall Freddie Mercury and Montserrat Caballé singing Barcelona song in front of the fountain.
It is a must-see if you happen to be in town. The Fireworks are coordinated with music and the water and light from the fountain. You can get a flavor of the event by watching this video from last year starting at minute 2.
I hope you have a good time if you decide to visit the city during this whole week.