For all you hard-edge bargain hunters out there, when are groceries the cheapest?

I know some stores often have deals between Mon-Wed that vanish if you turn up later in the week (esp on fresh goods).

But I also have a suspicion that the "specials" they offer on the week that falls on govt pay week tend to be less of the core grocery items and more discretionary items like chips, gourmet whatsits, and softdrinks, etc. The off-pay week seems to be more of the basic grocery items that will lure the budget-minded people back into the store.

What do you think of this theory? When are groceries the cheapest?

PS - in SA so please exclude Aldi from this discussion.