First, Sunday's during Lent are not recognized as a time for mourning since it is the day of the Resurrection and calls for celebrating. Friday being the day of Jesus' death on the Cross is the day for penance and mourning all year, and even more so during Lent.
Lent ends on Holy Saturday Easter Vigil. (Holy week starts with Palm Sunday and the Holy Triduum starts on Holy Thursday.) Without counting the Sunday's and Holy Saturday we have our 40 days this year.
There are many references to the number 40 in the Bible. Moses stayed on the Mountain of God for 40 days and the Hebrew's wandered the desert for 40 years. The spies were in the land for 40 days (Numbers 13:25). Elijah traveled forty days before he reached the cave where he had the vision. Nineveh had 40 days to repent.
The most important 40 is the 40 days Jesus spent praying and fasting in the wilderness. We are called to imitate Jesus' 40 days of prayer and fasting during Lent.
"'For we have not a high priest who is unable to sympathize with our weaknesses, but one who in every respect has been tested as we are, yet without sinning' [Heb 4:15]. By the solemn forty days of Lent the Church unites herself each year to the mystery of Jesus in the desert." (CCC 540).
