There are so many possible reactions to September 11, most of them embodying some flavor of anger. Anger has its place: anger moves us to work for justice.
But if we're going to commemorate September 11, I suggest that the most Catholic way to do this is with prayer and penance.
Let's pray for our nation, for all those who have been harmed - directly or indirectly - by the attacks of nine years ago.
Let's also pray for our enemies and bless those who persecute us. (As St. Paul reminds us, this heaps burning coals of charity upon their heads. He follows saying, "Do not be overcome by evil, but overcome evil with good." Romans 12.19-21) Let's pray for God's will to be done for them, especially that they may repent and find the fullness of Truth.
And let's offer whatever sufferings we bear, whether they're related to the attacks or any subsequent events or are purely personal, to God our Father in union with the sufferings our Lord Jesus bore on the cross. Let's do some act of penance today, in reparation for our own sins, and for the sins of all the world.
This suggestion goes against my personal inclinations; but one of the roles of religion is to correct and refine our personal inclinations. Today, whatever feelings arise in me, I'll try to put my remembrance into action through prayer and penance.