22 April

On 22 April 1945, the Soviet Army entered Berlin. Some reports held that Adolf Hitler then suffered from a nervous breakdown, but certainly he knew from this point that his own fate was sealed. (PROSE: The Shadow in the Glass)

5:02 PM on 22 April 2011, at Lake Silencio, Utah, was a still point in time, which made it easier for the Silence to create a fixed point in time.

On that day, River Song emerged from the lake in a space suit, and supposedly killed the 1103-year-old Eleventh Doctor before he was able to regenerate. In reality, the Doctor was in a Tesselecta during the murder, and remained unharmed during the whole process. (TV: The Impossible Astronaut, The Wedding of River Song)

The Doctor, at age 1103, recruited via numbered envelopes a 909-year-old version of himself (1), an older River Song (2), Amy Pond and Rory Williams (3), and Canton Delaware (4). All arrived in Utah and, except the younger Doctor, met up with the older Doctor. After his supposed death, they cremated his body on a boat and let it float away.

After cremating the Teselecta, River, Amy and Rory went to a diner where they encountered his younger self. They left in the Doctor's TARDIS to the Oval Office in 1969. (TV: The Impossible Astronaut)

Alternate timeline
An alternate timeline was created when River instead exhausted the weaponry of the astronaut suit, and didn't "kill" the Doctor. Immediately, a new reality came into being where all of history happened at once, and the time remained 5:02 PM on 22 April 2011 forever. In this reality, the Eleventh Doctor and River Song got married. The timeline was reverted when the Doctor and River, the opposite poles of the disruption, kissed, the proximity shorting out the differential, and letting time continue its course. (TV: The Wedding of River Song)

Behind the scenes
This was one of only three instances where a new series Doctor Who television story, namely TV: The Impossible Astronaut, took place on the same day as its broadcast: 22 April 2011. While the episode was actually aired on 23 April British time, this can be explained by the time difference between Utah and the UK, meaning that the Doctor was actually "killed" at 12:02 AM British time, the same day as the broadcast. The other two occurrences were 26 June 2010 and 25 December 2009.