On 12 April 1264, the master-builder Giovanni di Simone, the architect of the Camposanto, and 23 workers went to the mountains close to Pisa to cut marble. The cut stones were given to Rainaldo Speziale, worker of St. Francesco.
Cardinal Robert Bellarmine, one of the most respected Catholic theologians of the time, was called on to adjudicate the dispute between Galileo and his opponents. The question of heliocentrism had first been raised with Cardinal Bellarmine, in the case of Paolo Antonio Foscarini, a Carmelite father; Foscarini had published a book, Lettera ... sopra l'opinione ... del Copernico, which attempted to reconcile Copernicus with the biblical passages that seemed to be in contradiction. Bellarmine at first expressed the opinion that Copernicus's book would not be banned, but would at most require some editing so as to present the theory purely as a calculating device for "saving the appearances" (i.e. preserving the observable evidence). Foscarini sent a copy of his book to Bellarmine, who replied in a letter of April 12, 1615.
Support for the Rivera regime gradually faded, and Miguel Primo de Rivera resigned in January 1930. He was replaced by General Dámaso Berenguer, who was in turn himself replaced by Admiral Juan Bautista Aznar-Cabañas; both men continued a policy of rule by decree. There was little support for the monarchy in the major cities. Consequently, King Alfonso XIII gave in to popular pressure for the establishment of a republic in 1931 and called municipal elections for 12 April of that year. The socialist and liberal republicans won almost all the provincial capitals, and following the resignation of Aznar's government, King Alfonso XIII fled the country.
Preoccupied with military matters and spending most of his time at his military headquarters on the eastern front, Hitler came to rely more and more on Bormann to handle the domestic policies of the country. On 12 April 1943, Hitler officially appointed Bormann as Personal Secretary to the Führer. By this time Bormann had de facto control over all domestic matters, and this new appointment gave him the power to act in an official capacity in any matter.
On 12 April 1945, Roosevelt died, and despite their uneasy relationship de Gaulle declared a week of mourning in France and forwarded an emotional and conciliatory letter to the new American president, Harry S. Truman, in which he said of Roosevelt, "all of France loved him".
On April 12, in Shanghai, many Communist members in the KMT were purged through hundreds of arrests and executions on the orders of General Bai Chongxi. The CPC referred to this as the April 12 Incident or Shanghai Massacre. This incident widened the rift between Chiang and Wang Jingwei, another warlord who controlled the city of Wuhan.
On 12 April, the Belgian government, which was one of the largest troop contributors to UNAMIR, and had lost ten soldiers protecting Prime Minister Uwilingiliyimana, announced that it was withdrawing, reducing the force's effectiveness even further.
On April 12, Marco Marra, director of the Michael Smith Genome Sciences Centre, which is part of the British Columbia Cancer Agency, announced that scientists at his centre had broken the genetic code of the virus suspected of causing the disease.
Starting in 2004 a disarmament process was begun of Colombia's paramilitary groups (especially the AUC) and was completed on April 12, 2006, when 1,700 fighters turned in their weapons in the town of Casibare.
In the season finale on April 13, Bryant scored an NBA season-high 60 points against Utah in his last NBA game, outscoring the entire Jazz team 23–21 in the fourth quarter, in the Lakers' 101–96 victory. He became the oldest player to score 60 or more points in a game at 37 years and 234 days old. The Lakers finished the season with a 17–65 record, their worst record in franchise history.
The first alternative offered was that if MPs rejected May's deal in the next week, Brexit would be due to occur by 12 April 2019, with, or without, a deal—or alternatively another extension be asked for and a commitment to participate in the 2019 European Parliament elections given.