In some towns in the region, not wearing a costume can come with consequences, like having flour thrown at you. Dressing up is important!
Entroido is a celebration before the more serious period of Lent in the Catholic calendar. Lent is a 40-day period of reflection leading up to Easter. Many people in Spain are Catholic, so Entroido has a somewhat religious connection. However, Entroido also incorporates many long-lasting traditions that are separate from religion, and may even date back to the arrival of the Roman Empire in the north of Spain.
The ocean is one of the most important elements of Galician culture. Seafood is a staple in traditional Galician cuisine, with octopus, mussels and sardines all being important culturally. One tradition in the city of A Coruna, where I live, is the Parade of the Sardine to mark the last day of Entroido. A large wooden sculpture of a sardine is created in the city, then a multi-hour parade leads this sardine around the city in one last celebration before the beginning of Lent. The parade takes the form of a funeral procession for the sardine, with the sardine eventually being burned or buried at the end. The end of the parade is accompanied with fireworks and more celebrations on the beach, signaling the end of Entroido.