February 21 marks the beginning of the Triodion.
Triodion is a liturgical book and a period of the Church year which marks pre-Easter time. It precedes the Pentecostarion period and it comes after the longer period of the Octoechos. Thus, Triodion lasts from the Sunday of the Publican and the Pharisee and until the Great and Holy Saturday (the one preceding Easter): it has 10 weeks in total. This is a time when Christian Orthodox all over the world are called to prepare spiritually to meet the glorious feast of the Resurrection of our Lord Jesus Christ.
For Orthodox Christians, LENT begins on March 15th, 2021.