"Today"
We learn to pray at certain moments by hearing the Word of the Lord and sharing in his Paschal mystery, but his Spirit is offered us at all times, in the events of each day, to make prayer spring up from us. Jesus' teaching about praying to our Father is in the same vein as his teaching about providence: time is in the Father's hands; it is in the present that we encounter him, not yesterday nor tomorrow, but today: "O that today you would hearken to his voice! Harden not your hearts."
Source: Catechism of the Catholic Church (Vatican.va)