From the blog "Tools and Technology"

For many years, we have introduced changes to the Region simulators by deploying updates first to one or more of our Release Candidate (RC) channels, and rolling them on the Wednesday following the main channel roll on Tuesday. We evaluate the performance and stability in those RCs before making the changes to the rest of the Grid. This is an essential element of evolving Second Life because the size and variability of our virtual world are so great that there is no way we can test (or even know