The backbone of the organization of the 6 weeks of Great Lent is the system of readings and celebrations of the Saturdays and Sundays of Great Lent. The Gospel readings of the Saturdays and Sundays of Great Lent are taken from the Gospel of Saint Mark with one exception: the first Sunday of Lent, when the Gospel is taken from the Gospel according to Saint John. The Epistle readings are from Hebrews.
The reasons of this selection are obvious. As far the Gospels are concerned, Mark is the evangelist who presents Christ as the prototype of a Martyr, actually the unique, true and authentic Martyr. In view of the liturgical celebration of the passion of Christ during Holy Week, what else would be more suitable and appropriate than to read the Gospel that emphasizes the martyr character of Christ. As far as the Epistle readings are concerned, the Epistle to the Hebrews, is the Epistle that emphasizes the intercessional, mediatory and placatory role of Christ's self-sacrifice: "For Christ has entered not into a sanctuary made with hands... but into heaven itself, now to appear in the presence of God on our behalf. ... He suffered once for all anticipating the end of all the worlds, in order to put away sin by the sacrifice of Himself ... So, Christ having been offered once to bear the sins of many, will appear a second time, not to deal with sin, but to appear for their salvation to those who are eagerly waiting for Him." (Hebrews 9: 24-28 the Epistle reading of Saturday of the 5th week of Lent.)
Now, based on the readings, the Sundays of Lent are dedicated to a variety of themes, with which the spiritual stream of Lent flows naturally into the climate of Holy Week. Thus, according to the Gospel of the 1st Sunday, Philip calls Nathaniel to get to know Jesus personally. He calls him with the words "come and see". The words of Philip are obviously an allusion to the visual character of Holy Icons. The 2nd Sunday is dedicated to the double healing of the paralytic, through the forgiveness of his sins followed by the restoration of his physical health. The relationship between this Gospel passage and the penitential character of Lent is obvious! The 3rd Sunday is dedicated to the self-denial, self-sacrifice and Cross, not of Christ, but of every faithful person who wants to follow Christ. The 4th Sunday's gospel is the miracle of a healing of a demon-possessed young man. When the disciples asked Christ why they were not able to heal the boy, Christ in reply emphasizes the instrumental importance of Prayer and Fasting to the effect of getting rid of the evil powers. There is nothing more relevant to the struggle of Fasting and the insistence on Prayer that constitute the main aspects of Lenten spirituality. Finally the 4th and the 5th Sundays' readings contain preannouncements of Christ's voluntary death on the cross, whereas the 5th also contains the lesson of humility and self-offering as a response to the selfish and naive worldly query of the disciples about "who is greater" among them. Let us add here that all these Sunday Gospel themes are the continuation of the fundamental ideas proclaimed through the Gospel readings of the first 4 Sundays of the Triodion, the most significant among them being that of limitless love towards neighbors, which is expressed as an unconditional gift of forgiveness.