The "eve" in question is the start of the Crimean war. The setting, however, is provincial Russia (as usual in Turgenev's work) and the characters are a small circle of close friends. Yelena is a serious girl who needs an ethical and active life, and finds a freedom fight to throw herself into -- Bulgaria from the Turks. Insarov, Yelena's Bulgarian hero, is astutely mocked by other participants.
