Shloshim, which means thirty in Hebrew, are the thirty days following the burial, with the day of the burial counting as the first day. Usually then, Shiva is the first seven days of Shloshim. As with Shiva, some festivals affect the Shloshim period and your rabbi will advise you as to how a festival impacts your particular situation.
At the conclusion of Shiva, Shloshim serves as a period of re-entry into the world of the living for the mourner. This is the time when the mourner returns to work or school, and begins to start living without their loved one. During Shloshim, the mourner traditionally avoids music, gaiety, and other forms of celebrations. Your rabbi will help you with specific questions that may arise such as what happens if a previously scheduled wedding or Bar or Bat Mitzvah occurs during the Shloshim period.