My theory:
Since IOTD has no new images on Sunday, we are all starved on Monday to catch up, therefor more likely to log in and check it out...
Absence makes the heart grow fonder.
I never check in on the weekends (Rat Cave has no internet access), so usually on Monday my to do list looks like this:
#1 check Cellar,
#2 Check Post Secret
#3 Check work email
#4 Figure out what I forgot to do last week and get to work....