Actually, the ever-helpful lentesta from the staff of the Unofficial Guide lists Saturday as one of the better days to visit the MK. This is contrary to what I always read on these boards so I PM'd Len to question why he thought Saturday was a good day. Here is the response that he sent me (I hope you don't mind me posting this Len)
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Hi Mike,
Thanks for writing! Lots of folks seem to think that Saturday is a bad day to visit the MK. But the data we've collected don't seem to support this. Our suggestions are based on the data we've collected in the parks over the past few years. I'm not sure what other folks use to make their recommendation, but my guess is that it's something more subjective.
As to why Saturday may be less busy at the Magic Kingdom, there are two popular explanations. One is that Saturday is an EMH day at the Studios, so many Disney resort guests will go there. A corollary to this may be that Disney resort guests may be waiting until Sunday to head to the MK.
A second reason for this may be that Saturday is a travel day for many folks, especially those who drive. Last time I checked, more visitors to WDW drive than fly. That's just a guess.
Hope this helps. We're constantly re-evalutaing the our recommended days to visit each park. If you'd like to e-mail me before your trip, I'll be happy to share our latest research with you. Thanks again for writing!
Len