Correct, Sunday is the first day of the week and started the previous 6pm. Jesus had already been resurrected for some time.
Matthew 27:2-5
There was a violent earthquake, for an angel of the Lord came down from heaven and, going to the tomb, rolled back the stone and sat on it. His appearance was like lightning, and his clothes were white as snow. The guards were so afraid of him that they shook and became like dead men. The angel said to the women, “Do not be afraid, for I know that you are looking for Jesus, who was crucified.
Notice the guards were there when Mary arrived, in fact, Jesus was still there also.
The guards were still there and were frozen with fear.
Looks as though the event occurred just before Mary arrived.