The judge in the Boston Marathon bombing case on Thursday set June 24 as the date he will formally sentence Dzhokhar Tsarnaev, the convicted bomber, to death. A jury on May 15 recommended he be put to death; the official sentencing next month, by Judge George A. O’Toole Jr. of Federal District Court, will complete that phase of the process. But defense appeals are likely to extend the case for years, and there is no telling when or even whether an execution might take place. Twenty survivors and relatives of victims of the bombing are expected to speak at the sentencing hearing. Mr. Tsarnaev, 21, may also speak if he wishes; he did not testify during his trial.