It is still the 'first day of the week' because the Creation Story of Genesis is still being re-written. Early in the morning, the disciples have come to realise that Jesus is with God, (John, 20:1-18). Now, on the evening of the same day, they find that He is always with them. He is the bridge, linking heaven and earth. This is who He was, who He is and always shall be.
Notice that He does not enter through doors, or appear as he used to before his Resurrection and Ascension. He is 'there' in the very heart of them, calming their fears and giving a Peace and a Joy that cannot be taken away.
But He has to offer it twice! The Beloved Disciple had to look twice into the empty tomb before he came to belief (John, 20:8). Mary of Magdala had to turn twice before she recognised the true identity of the 'gardener' (John, 20:16). Now He gives them His Peace. First, to free them from fear, and Twice to strengthen them for their mission.
So, what is given must be given away so that an unending thread of love is spun from the Father, through the Beloved Son and in the Spirit, to every human heart.
Just as God breathed life into the clay of the earth and made Humanity, so now, our Risen Lord breathes into His disciples the Gift of the Spirit. Living here they move with the Spirit to make all things One. How? In the only way possible, through the work of Forgiveness and Reconciliation.
At the start of His Mission, Jesus unrolled the scroll of Isaiah and saw Himself perfectly reflected in the words, "The Spirit of The Lord is upon me. Because The Lord has anointed me. He has sent me to bring good news to the poor, to proclaim liberty to captives, to heal the broken hearted and set the prisoner free. To proclaim a time of Jubilee from The Lord ......"
In Sacred texts, Signs of the presence of the Spirit are variously described as : Wisdom, Understanding, Counsel, Courage, Knowledge, Piety, Reverence, Faithfulness, Prophecy, Discernment, Healing, Miracles, Speaking in Tongues and Interpreting them, Love, Joy, Peace, Patience, Kindness, Gentleness, Goodness, Long-Suffering, Humility, Modesty, Continence and Chastity.
But, today's Gospel underlines that all of these are doorways into the One Gift and Work of the Spirit. Forgiveness and Reconciliation. Once we are united, the Spirit flows through us into the world, that we and God Love without regret.