Quote from Martinghoul:
As a TRADER (caps yours), I like to think about the roll, as opposed to doing it on a specific date. The roll is a TRADE (caps mine), which can get quite volatile, so I would do it when it is likely, in my view, to make me money. I would never do anything as silly as always rolling on a specific date pre-determined in advance.
Now, there are several inputs into the decision-making process, such as repo on the two CTDs, the open interest, etc, but that's a discussion for another day.