Among those taking the stand: a forensic technician, Valencia's mother and a female friend who was with Valencia the night he died.
In court, prosecutors showed, in its entirety, surveillance video taken from the home. It shows Valencia being shot by a man wearing a hoodie.
The gunman was Narcisco Gatica, prosecutors say. He and his friend, Berly Valladeres, were turned away from the party at the Logan Square home and later returned with a gun to exact their revenge.
In a statement last year, police said Valladares admitted to detectives that he supplied the gun to Gatica.
Family and friends of Valencia said they will be in court everyday through the course of the trial but will not comment publicly until afterward.
After the shooting, family and friends were outspoken of Valencia's exemplary qualities as a friend, student and leader.
"He was one of the most kind and loving people; kind to everybody. He was kind to his best friends. He was kind to the cashier. He was kind to the homeless person on the street. Everyone meant something to him," Valencia's friend, Sarah Rothman, said last year at Valencia's funeral.
The trial resumes Monday.