Let's look at it in reverse. What are the best days to go to
Disneyland--(1) Wednesday; (2/3) Tuesday/Thursday.
Friday, Saturday, & Sunday are not a good choice for DL.
Weekends are not good anywhere.
With all that in mind,
Monday would be the best for Universal Studios. I think people actually look at Tuesday being the best Universal day, but I'm not willing to give up one of the three best days at DL for it. From what I've learned on Disboards is that if you get to Universal when it opens and tour in the right order, you will be done with everything by 3:00 PM.
Have fun.