... is (or was) actually the 19th of April. According to Wikipedia, Pathé News recorded commemorative events in 1916, including a wreath being laid at Disraeli's statue outside Westminster Palace; but I can find no evidence of its being celebrated at any later date.
Benjamin Disraeli died on 19 April 1881, aged 76. The primrose was said to be his favourite flower; Queen Victoria (again according to Wikipedia) would often send him bunches of them from Windsor and Osborne House. She sent a wreath of primroses to his funeral, with a handwritten note: "His favourite flowers: from Osborne: a tribute of affectionate regard from Queen Victoria". According to the Daily Express however, "it has never been clear whether she meant Disraeli's favourite flower or her late husband Prince Albert's."
The Primrose League was established in 1883, and named in Disraeli's memory. Its aim was to spread Conservative principles in Britain; Lord Randolph Churchill (father of Sir Winston) was a founder member.
© Macclesfield Quiz League 2018