Yah! it is true and this truth is because of our belief, rituals and Holybooks said the same. Every Day is just a DAY nothing else until it is having a name which is because of a some special, logical and scientific reason. As we having only 7 days in a Week, we having the name of every day with most respectable planets and God .
And About Sunday (Sun+Day) wherein SUN is our biggest planet and most respectable and that is actually controlling our Solar System and other planets
Akaro
