Many others will speak far more eloquently about today than I could, so I’ll just mention several brief thoughts.

I found out about the 9/11 attacks after voting in the Democratic primary (which was cancelled). At that time, it wasn’t clear what had happened: the radio reports were describing it as a small plane–making it sound like a horrible accident.
When I arrived at work, we turned on an old banged-up TV where it was clear what exactly had happened. Then the reception cut out. Because the tower with the TV transmitters fell.
There’s one other thing. A colleague suggested that Iraq might be behind this (that’s what a lot of TV pundits were saying). I can’t remember exactly how I phrased it, but I remember I was angry and said that it wasn’t Iraq, because that would make no sense, and that it was Al-Queda who did this. What an awful foreshadowing.
Three days later, I was home in DC for Rosh Hashanah, and I remember going to the Smithsonian–which was like an empty mausoleum. On the Mall, there were anti-aircraft missile batteries. That week, I had reason to drive by the Pentagon, which also was ringed by air-defense batteries. And on 9/11, when I saw pictures of the ambulances waiting to enter Arlington Hospital (unlike New York, there were a lot of injured), they were lined up waiting to enter the emergency ward entrance on roads that I routinely travelled when I lived in VA.
I still remember that day very clear. What an awful day it was.