External Sacrifice:
So why exactly are you not eating candy for the next month?
This is a very popular penance during Lent, and the questions about it are just as popular. Heaven forbid you should avoid the cake at the birthday party! You LOVE cake! But it’s Lent – a time for penance and sacrifice. Christ has said, “If anyone wishes to come after me, he must deny himself and take up his cross daily and follow me” (Luke 9:23). Giving up something we enjoy strengthens our love for Christ and our resilience against temptation. As Pope Benedict XVI said in his Lenten address of 2009, “Through fasting and praying, we allow [Christ] to come and satisfy the deepest hunger that we experience in the depths of our being: the hunger and thirst for God.” It unites us with Christ’s own sacrifices and gives us a deeper appreciation of the blessings in our lives.
External Sacrifice vs. Internal Conversion:
I don't get it --- All you do is give up candy?
Now, here is an interesting question. External sacrifices are the most obvious kind, so sometimes it does look like all we do is avoid chocolate. However, if you look closer, you'll realize that fasting and abstinence have always gone hand in hand with two of their best friends: prayer and almsgiving. External sacrifice is a manifestation of interior conversion: "…interior conversion urges expression in visible signs, gestures and works of penance" (CCC no. 1430). Interior conversion is where prayer and almsgiving come into play. In almsgiving, we show mercy and generosity to others, giving them a chance to experience the blessings we have. In prayer, we are communicating with God, asking him to bless and perfect our fasting and almsgiving: "… prayer is the living relationship of the children of God with their Father who is good beyond measure, with his Son Jesus Christ and with the Holy Spirit" (CCC no. 2565). It's not just avoiding sweets. It's glorifying God by growing in his Love.
source: United States Conference of Catholic Bishops website: http://www.usccb.org/prayer-and-worship/liturgical-year/lent/ideas-for-evangelization-during-lent.cfm