The AMC period drama “Mad Men,” which returns for its fourth season on July 25, has consistently woven episodes around actual historical events. Matthew Weiner, the series creator, has said that he wanted to take a classic 1950s archetype, embodied by the character Don Draper, through the upheaval of the 1960s and into the early 1970s. On this website, created by the New York Times, a look at the historical context of the series and at events that could shape it as it progresses.