I had the same experience with the Hilton Mission Bay Resort...no Sat. night only reservations. We eventually changed our plans and will arrive on a Sun. and stay for 5 nights instead of 1.

Interestingly, I found that if I booked a 5 night stay with a Sat. arrival instead, the price was about $80/night higher on Sat. and $40/night higher for the next 4 nights, than what we're paying for our stay!