13 July being independence day, celebrating the recognition of the country by the Great Powers in 1878, and the start of the resistance against the Nazis in 1941.
Inside was this envelope:
"nezavisnost" means (not surprisingly) "independence"; "prvi dan" means "first day".
All is explained on the back of the envelope:
The latter two images are a little bigger than the original - the first one, about the same size.