For me, it's more about the time of the day I'm running and the distance of the race. If I'm keying on a relatively early morning race, I want to do some running at around race pace for shorter races or of long distances if keying on a longer race. For later morning races, I want to do the same thing but later in the morning. Since my weekday runs are late morning runs and my weekend runs are more early morning, I plan things according to that schedule.
Basically, my body doesn't care what the day of the week is but it does get into a routine in regards to time of day where (most often) it may not like to run fast early in the morning.