The 1956 World Gliding Championship - St Yan, France 

you are right

I think this was in 1957,because I went there .You should be right.

Its could be a 1956 because prototypes were produced in spring of this year. 1958 is start of mass production in Let Kunovice.

And another comment after some research, Blanik OK-6201 was the first prototype. Ok, I found czech text which describes fate of the first two Blaniks (OK-6201 and 6202), both were damaged and actual OK-6201 from video was created by their merging and it was send to France in 1956.