Julian Fellowes has woven the timeline of Downton Abbey to actual events of the day. In "The London Season," the year is 1923, when Lady Rose MacClare is presented to King George V and Queen Mary by Countess Grantham, her aunt Lady Cora Crawley. She is shown with several friends, including Mrs. Freda Dudley-Ward, who in reality had been involved with The Prince of Wales (for over 5 years since 1918), referred to him as "David" (his actual name was Edward Albert Christian George Andrew Patrick David Windsor, but was called just David by his close friends and relatives) and who would have been 29 years old at the time of this Presentation. Mrs. Dudley-Ward refers to the Prince of Wales a "...a faithful little chap" who 13 years later becomes King Edward VIII upon his father's death, and abdicates the throne in 1937 after only 326 days as King so he can marry Wallis Simpson, a twice-divorced American. (See Duke of Windsor and Duchess of Windsor.) They remained married for the next 35 years, until his death in 1972. There was actually a situation of inappropriate love letters to/from a former lover involving the Prince of Wales while he was seeing with Mrs. Dudley-Ward.
When James concedes that he doesn't "have the right to mind" serving at the picnic Carson responds sarcastically "thank you Wat Tyler." Wat Tyler was a leader of the Peasants' Revolt of 1381.
Edith mentions that there are rumors that her boyfriend Michael got into a fight in Munich with some aggressive men wearing brown shirts. These men were members of the nascent Nationalist Socialist (Nazi Party) that would seize control of Germany 10 years later.
Rose meets Madeleine Allsop who is friends with Freda Dudley Ward. James Fox who plays Lord Aysgarth is connected to the real Freda Dudley Ward. His brother was married at one point to Fredas granddaughter.
Martha Levinson states "I own what is considered a cottage on Bellevue Avenue." Bellevue Ave in Newport, RI is home to one of the largest concentrations of Gilded Era mansions in the United States. Most were built as summer homes, aka summer cottages, for families such as the Astors and the Vanderbilts. The more well-known estates include The Breakers, The Elms, Marble House, and Rosecliff, among others. The area is now listed on the U.S. National Register of Historic Places and many of the estates are open to the public as museums.
Alastair Bruce: the series historical advisor briefly appears as Lord Chamberlain at the Royal reception, announcing the guests to King and Queen.