So last night I went to CVS - the second week in a row I showed up after 6pm in the evening on a SATURDAY....and when I mentioned something about the ad in last weeks paper he said to me "I'll tell you a secret...did you know these ads run from Saturday at 6pm to Saturday at 6pm?" So - a few things I figured out...last week I had something scan at the right price but no ECB came out, I caught it and they issued it to me; this week it worked. If you go on Saturday night you will notice that a lot of the ECB and sale tags have been removed already, making it trickier to find the right products BUT both ads are valid for the hours between 6pm and close.
I suppose if you go to Iheartcvs and look ahead to the next weeks ad, you could take advantage of deals sooner than Sunday. I would just make sure you are watching everything closely to make sure both computer systems are doing what they are supposed to be doing :)