Short answers, pulled from the story.
Saint George's Day is normally celebrated on the 23rd of April, the traditionally accepted date of the saint's death during the Diocletianic Persecution. However, when this date falls between Palm Sunday and the Second Sunday of Easter, it is transferred to the following Monday in Anglican, Catholic, and Lutheran calendars. Eastern Orthodox churches that use the Julian calendar, including the Serbian Orthodox Church, observe the feast on the 6th of May.
Saint George's Day is observed in England, Ethiopia, Georgia, Catalonia, Aragon, Palestine, Rio de Janeiro, Alcoi, and Genoa, where he is the main patron saint. Albania, Bosnia and Herzegovina, Bulgaria, Greece, Lithuania, Portugal, Romania, Serbia, Syria, and Lebanon also mark the day, as he is an important patron saint in those places. Bulgaria and Newfoundland and Labrador observe it as a public holiday.
The earliest documented mention of Saint George in England comes from the Venerable Bede, who lived from around 673 to 735. Edward III, who reigned from 1327 to 1377, placed his Order of the Garter under the banner of Saint George, and declarations by the Province of Canterbury in 1415 and the Province of York in 1421 elevated the feast to a double major, making church attendance mandatory. His association with England was reinforced by soldiers invoking his name as a battle cry during the Hundred Years' War.
In Catalonia, the 23rd of April is known as the Diada de Sant Jordi and is celebrated with an exchange of books and roses. Traditionally, boys gave girls a red rose and girls gave boys a book; today the exchange goes both ways regardless of gender. In 2015, 1.5 million books were sold across Catalonia on that single day.
In Hungary, Saint George's Day on the 24th of April is linked to magical practices, including ritual dew-collection believed to multiply butter, and the warding off of witches with willow twigs, incense, and onion garlands. Cattle were driven over chains and axes to charm fertility, and blessed poppy seeds were sprinkled around horses to protect them from the Szépasszony spirits. Shepherds were contracted on this day and offered eggs, szalonna, wine, and pálinka.
In Stoker's 1897 novel, Saint George's Eve is presented as a night when moroi, witches, and dark creatures must gather all their power before midnight. The date given in the book, the 5th of May on the Western Gregorian calendar, corresponds to Saint George's Day as observed by Eastern Orthodox churches at that time. A local woman warns the narrator that at midnight 'all the evil things in the world will have full sway.'