From the point of two days before the arrest of Christ, the narrative now moves back from the Tuesday to a point six days before the Passover (John 12:1), before Christ actually entered Jerusalem. Mark says – he doesn't exactly use these words, but he in effect says – ‘Now, for a moment, something happened which I want you to know about.
Why is this account moved out of place in two of the Gospels? Why suddenly bring it in when we are talking about the Tuesday before Calvary? Surely because of the tremendous contrast between what Mary did and what Judas did. There is the event of Mary’s anointing of him, and then follows the treachery of Judas, so that is undoubtedly the reason.