Someone please correct me if I am wrong, but...
It has been my experience that the game picks the dates, and puts all of your races on Saturday when you configure a custom series. I think the only way that you could possibly get a race to show up on a different night of the week would be to set up a series with over 52 races (since there are 52 weeks in a year), but you would then have a race every Saturday of the year before a race on another night would be possible. To my best knowledge, the series STR files do not have variables for nights of the week, and the computer will spread your schedule across the calendar indiscriminately.
This thread makes me want to experiment with a series with over 52 races on the schedule to see what the hell will happen. I will let you know if I think of something else, but I am not sure if what you are talking about is possible.