As I discovered, it was also the day that the business-wide reorganization hit many of the people who sit in the cubes around me. I'm not in their group any more, but I used to be and worked for many years with most of them.
One by one, people were called in to talk with the second level manager and learn their fate. Everyone, regardless of the result, was told they were free to take the rest of the day off. During the day, there was a lot of people standing around cubicles talking (myself included, as I caught up with folks after my mostly-absent summer).
In the end, around 10-20% of the people were told that there wasn't a place for them in the new organization. As people found out the result, they left, and the cubicle farm got quieter and quieter. By late afternoon, it was only me and a few stalwarts sitting in an ocean of empty cubes. A very weird day, indeed.
ETA: BTW, my job is fine. I found out my project and I survived the reorg about a week and a half ago....