The Triodion begins on the Sunday of the Publican and the Pharisee, which can fall as early as January 11 and as late as February 14.
Great Lent begins on Clean Monday (which follows Cheese-fare or Forgiveness Sunday), and it ends on the Friday before Lazarus Saturday, which is the day before Palm Sunday.
Holy Week begins with Lazarus Saturday and ends on Holy and Great Saturday.
The Triodion therefore comprises not only Great Lent, but the three preparatory weeks before, and Holy Week as well.
The hymns of the Menaion continue to be used with the Triodion until Lazarus Saturday; after that only the Synaxarion of the Menaion is used daily.