Originally Posted by SWTOR
Hi everyone,
We will be bringing down servers for four hours on March 23rd, 2012 from 2AM CDT (12AM PDT/3AM EDT/7AM GMT/8AM CET/6PM AEDT) until 6AM CDT (4AM PDT/7AM EDT/11AM GMT/12PM CET/10PM AEDT) to deploy a patch. All game servers will be offline during this period, and the deployment is expected to take no more than four hours.
Upcoming Patch
Date: Thursday, March 23rd, 2012
Time: 2AM CDT (12AM PDT/3AM EDT/7AM GMT/8AM CET/6PM AEDT) to 6AM CDT (4AM PDT/7AM EDT/11AM GMT/12PM CET/10PM AEDT)
All game servers will be offline during this period. This deployment is expected to take no more than four hours.
Thank you for your patience as we maintain service for Star Wars™: The Old Republic™.
(source)
No info on what the patch will actually contain yet.