harry potter - In which year did Dumbledore become headmaster?

In which year did Dumbledore become the Headmaster for Hogwarts school of Witchcraft and Wizardry?

Answer

Although the generally accepted date is 1956 (e.g. presuming that Minerva McGonagall replaced Dumbledore as Transfiguration Mistress on his appointment to Headmaster) there's a fair argument that he didn't actually become Headmaster until late 1964/early 1965.

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  We know from the latest release on Pottermore that Minerva McGonagall was appointed as Dumbledore's assistant in 1956 but not as his replacement. There's no canon description of how long she worked under him but we can reasonably assume it would take quite a few years to go from a 21-year-old trainee to full Mistress at Hogwarts;

  
  

  
  

  She sent an owl to Hogwarts, asking whether she might be considered for a teaching post. The owl returned within hours, offering her a job in the Transfiguration department, under Head of Department, Albus Dumbledore. _{[Pottermore website]}

  
  

  
  


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  Lupin was born in 1960 and didn't become infected until 1963/4. He speaks of the change in attitude at the school when Dumbledore became Head.

  
  
  

  
  

  I was a very small boy when I received the bite. My parents tried everything [edit] It seemed impossible that I would be able to come to Hogwarts. Other parents weren't likely to want their children exposed to me. But then Dumbledore became Headmaster, and he was sympathetic. He said that, as long as we took certain precautions, there was no reason I shouldn't come to school. ^{Prisoner of Azkhaban}

  
  

  
  

You can see an extensive discussion of this alternate hypothesis here.