I could not find the date of issue of this stationery, neither could I find the occasion for which this cover was issued. For sure it is related to sport. I’m not very familiar with Cyrillic alphabet, but I could find out that the word written in white and blue letters on the top of the illustration means “sport” .
What I liked is that the illustration re-uses the design of three different stamps issued by Belarus. The first one, starting from the left, is a stamp issued on the 25th of October 2001 about windsurfing. Here is a picture of the original stamp.