Rollover day today (Thursday 11 September)

Just a reminder to change from the U contract to the Z contract on software and trading platform as today is rollover day on US futures.

The last trading day is the 3rd Friday of the expiration month; rollover day is the Thursday one week and one day before the last trading day.
Sorry if this is a stupid question or if it has been answered before, but what is a "rollover day" and what are the repercussions if any?
DaxT, yeah you are right- for SP and other US financials- Dax trades right to the end, doesnt it-

JeanM- basically if u are trading futures, the SEP contract on the US indicies is coming to an end and the Dec contract ( code Z instead of the U ) is about to start-

If u watch the SP volume today u will notice the drop .....

Skimble- thanks for the reminder !! I must say I knew the date but had forgotten
This is a message I've just received (all times are US ET):

Today all US Financial markets will be observing four separate moments of silence in remembrance of the 9/11 attacks. The following minutes will be observed, 8:46, 9:03, 9:59, and 10:29 EST. In addition to these minutes, the NYBOT and NYMEX will close for an additional 21 minutes on the last moment of silence for the 21 members of the NYBOT & NYMEX that perished on 9/11. While it is encouraged that all trading cease during these times, GLOBEX trading may continue outside of the exchange.
yeah wierd isn't it, US stop a week prior to expiry, everyone else in Europe trades front month indexes into expiry day - Dax, CAC, ESTOXX, FTSE etc.

Bunds stop trading front month around the turn of the expiry month.