Officers found Casillas' body after responding to a call reporting a stabbing in the 900 block of Dakota Drive at about 10:40 p.m., according to San Jose police.
She was pronounced dead at the scene.
A suspect has been arrested in connection with the case, police announced Sunday. The suspect's name was not released.
Investigators said that Casillas and the suspect knew each other, but did not specify how.
The motive remains under investigation.
Anyone who might have information about the homicide is asked to contact the San Jose Police Department's homicide unit at (408) 277-5283.