Sometimes you add the "did" for emphasis. If someone says, "That could never happen" you might answer, "It did happen last year!" (With the stress on "did") Saying "It happened last year" has the same meaning, but is a more straightforward statement of fact. 

  You are correct  Natalia. "It did happen....";   is an odd expression which adds emphasis as Karen shows.

  To say  "it happened"  is by far the more convenient  expression, because as you know,

it is straightforward.

Thanks Karen and Bruce for your great help. Now I understand the differences. :)