The Code of Canon Law (can. 1248) permits Catholics to fulfill their Mass obligation by attending Mass “in the evening of the preceding day.”
Pope Pius XII, in the apostolic constitution Christus Dominus, set the earliest hour for such a Mass at 4 p.m. Until the Church issues a new rule regarding when to officially define “evening” in canon 1248, 4 p.m. remains the earliest time a Mass is considered to fulfill the next day’s obligation. https://www.catholic.com/qa/how-early-can-the-saturday-vigil-be