In today's readings we are told about fasting. Isaiah tells us the kind of fast that is acceptable to the Lord; a fast that brings justice, feeds the poor and shelters the homeless.
If you do this your integrity shall go before you.
In the gospel, Jesus tells the pharisees that his disciples do not fast because he is still with them but the time shall come when he shall be gone and then his disciples shall fast.
We fast now because we want to be part of what the Lord has done for us. We remember the days that are past; the times when we felt close to god, when he helped us through troubles and when he sent us aid of whatever kind we needed.
The fast of lent is the fast of longing; longing to play our part in the building of God's kingdom and longing for the strength to do so when the time is right.
The two readings together give us a pretty accurate coverage of the whole idea of fasting. But it is the actual fasting that is the hard bit!