The Purim of Saragossa
In 5180 the rabbis of Saragossa, Spain, decided that it was a disgrace to bring a Sefer Torah before the king every time, and chose to empty the Torah case on such occasions, (in Sephardic sifrei Torah the scroll may be removed from the surrounding case and one cannot tell if the scroll is inside the closed case except by its weight). A Jew who left his faith uncovered the deception and told the king, who chose to surprise the Jews by demanding them to open the case upon his visit.
On the night before the royal visit the Shamash had a frightening dream in which he was commanded to swiftly return the Sefer Torah to its case. He woke in alarm and, telling no one, ran to do what was demanded of him. The next day the king arrived and all the Jews greeted him with the Sefer Torah as usual. The king smiled and asked to see the Sefer Torah and the rabbis paled and trembled while only the Shamash remained calm. He gently opened the case and the king, shocked as much as the rabbis, realized he had been fooled. The 17th of Shevat was thereafter celebrated as the Purim of Saragossa, to be remembered each year
(The whole story appears in Luach Davar Be'lto).