"In that day."
What day is Jesus talking about? He is talking about the day when he has been raised from the dead. He is talking about the day when he ascends to heaven. He is talking about the day when he makes you one with the Father, just as He is. He is also talking about the day when He sends the Helper, the Spirit of truth, to us.
"In that day", is the day that the disciples were going to be going into soon. How do we walk in, abide in that day today? By walking in the resurrection life of Jesus.
Resurrection is preceded by death and burial. Christ-followers, in life, must go through death, burial, and resurrection; in Christ. We cannot skip death and burial, but we must go through them, in how we live our lives in order to walk in or abide in Jesus' resurrection life.
To get to resurrection life, we must lose our lives, take up our crosses and follow (Mark 8:34-5). We must share in his sufferings (Phil. 3:10-11). Resurrection life is a life that is hidden with Christ in God (Col.3:3).
"In that day, you won’t ask me anything."
Our spirits don't have questions, because in our spirits we are one with God. Our minds, our fleshly minds that are in the life-long process of dying and being crucified and being renewed and sanctified; these minds of ours, the same mind we use to make choices in our lives - that mind of ours has many questions for God and many things are hidden from that mind.
"I assure you that the Father will give you whatever you ask in my name."
God will give us whatever we ask in Jesus name. We have learned to tag our prayers with, "in Jesus name, amen", whether we're praying before a meal or praying for someone who is sick. You probably realize that saying, "in Jesus name", is not magic. Then, what is praying in Jesus name? It is to be immersed in Jesus. One with Jesus. Intimate with Jesus. Unified with Jesus. Congruent with Jesus.
"Ask and you will receive so that your joy will be complete."
Praying in Jesus name means that the person praying is immersed in Jesus. It is descriptive, not prescriptive. It is more something that we are, something that Jesus wants us to become, rather than a password. Jesus is saying that this is the place in prayer that he has for us.
"In that day" prayer, prayer "in Jesus name", are Jesus' prayers. I am one with Jesus and I have his mind in my heart, resonating with my spirit and I pray Jesus' prayers. What is Jesus interested in praying through me and in me?
We are living "in that day", the day after the cross, the day after the resurrection, the day after the ascension, and the day after the day of Pentecost. Today is the day of abiding prayer.
If we are not in Christ, we need to go to the cross, maybe again. The cross is the way in. The cross is also the way on. We might need to take up our crosses. We might need to die to our selves. We might need to go to the ascension and we might need to experience Holy Spirit baptism, perhaps again.