This hasn't always mattered to CA. In the 2010's, there were two Sydney tests starting on a Sunday (2010 & 2016), one on a Monday (2011) and three on a Tuesday (2012, 2015* & 2017).
* the 2015 started on January 6 as opposed to January 3 which was the normal start date for Sydney tests in 2010's.
2015 was changed to due Phil Hughes passing away and test schedule had to reworked in last minute.
They has to squeeze the postponed brisbane test a week before christmas. So they pushed the Sydney test back a few days for extra rest for players as they practically played 3 tests in 21 days.