I doubt it will become a holiday. It will be remembered vividly by those of us that lived through it that day for the rest of our lives however for children born on September 12, 2001 it will be recalled as a historic event of major significance and impact on his or her parents life much like Pearl Harbor is for baby boomers. I have seen over my lifetime the decrease in the amount of press and time spent on Pearl Harbor as the WWII generation continues to pass from this world. I suspect that much the same thing will happen with 9/11 as those of us that directly experienced it and lived through it pass.
For a bit of perspective the single most bloody event this country has ever been involved in is our own Civil War. In September 1862 the Battle of Sharpsburg (Antietam) was fought with losses of 2,108 Union soldiers, 9,549 wounded and 2,700 Confederates and 9,029 wounded. This was actually the single most bloody day in American history and we barely pause to think of it 140 years after the fact.