The Spanish region of Catalonia is scheduled to hold a referendum on independence on Sunday, despite fierce opposition from the central government in Madrid and from the courts, which say the vote would violate the Constitution. The unity of the country is at stake, as is the political survival of Prime Minister Mariano Rajoy. He has stepped up pressure in the region, which has seen arrests and the confiscation of campaign materials. Some Catalan leaders have suggested that Mr. Rajoy is taking the country back to the dark days of dictatorship, although he has resisted hard-liners’ calls for him to seize complete administrative control of Catalonia. Mr. Rajoy has fought off Catalan pressure before, including five years ago, when Spain’s economic crisis, and Catalonia’s tax contributions to poorer regions, bolstered the secessionist movement. But separatists in the region today are determined to press ahead. Here’s a guide to the vote, and to the events that led up to it.