These pancakes are Russian, and were (it's no longer officially recognized) eaten at the revelrous pre-Lenten feast known as Maslenitsa, or "butter week," from the Russian word maslo, butter. Along with eating blini, the Russians threw themselves into this festival with great exuberance. Many of the traditions associated with this time date back to pre-Christian revelry when they were, as we say in New England, trying to "break the back of winter." They'd build great ice hills upon which they'd haul effigies of winter, which were then sent crashing down the slopes. Huge bonfires were built to drive winter away. They even wrapped themselves in animal skins and made fearful noises to scare away evil spirits.
Although all of this can be traced back to superstition about the sun and the passing seasons, most of it became an excuse to break up the long, dark winter days with some fun. What the Russians loved best during Maslenitsa was eating blini, which they did with great abandon before the beginning of the Lenten fast. No meat was allowed during this week, thus the relish with which they consumed dairy products and fish.