She wounded eight children and one police officer and killed two adults in a shooting spree at Cleveland Elementary School in San Diego, California, on January 29th, 1979. The school was across the street from her house. She used a rifle she had recently been given for her sixteenth birthday. When the six-hour incident ended, she shrugged, "I don’t like Mondays. This livens up the day." She also noted that There was no reason for it, and it was just a lot of fun; It was just like shooting ducks in a pond; and [The children] looked like a herd of cows standing around, it was really easy pickings.
Convicted of two counts of murder and assault with a deadly weapon, she was sentenced to prison for 25 years to life. She is one of the only known female mass murderers in United States history, and may be the only one who was not a mother killing her own children.
Spencer's crime, lack of remorse, and inability to provide any non-trivial explanation for her actions when captured inspired the song I Don't Like Mondays by the Boomtown Rats, which was played incessantly on Monday mornings by album-oriented rock format radio stations in the United States throughout the 1980s.