The offers in a supermarket's weekly advertisement were agreed with suppliers months earlier. That lead time explains both the rhythm of promotions and their limits.
Promotions are negotiated, not decided weekly
A supplier wanting a featured position contributes to the cost, either through a lower wholesale price for the promotional period or through a direct payment for placement.
Those arrangements are settled in planning cycles that run well ahead of the selling week, because both sides need to commit production and space.
By the time the advertisement is printed, the price, the volume and the display position have all been fixed. Very little is decided in the week itself.
Production and logistics require the notice
A promoted line can sell several times its normal volume. Manufacturing that surge requires scheduling factory time and raw materials in advance.
Distribution centres also have to hold the extra stock and move it to stores before the offer opens, which cannot be arranged at short notice.
This is why a genuinely popular offer that sells out is rarely extended. There is no additional stock available, and the next slot is already committed to something else.
Categories rotate on a fixed cycle
Retailers avoid promoting the same category every week, since a permanent discount trains shoppers to wait and erodes the ordinary price.
Instead each category returns to promotion on a cycle of a few weeks, long enough that shoppers do not plan around it but short enough to keep interest.
Noticing the cycle length for the products you buy most is the practical use of this. A missed offer on a storable item usually returns within a predictable span.
Seasonal blocks dominate the calendar
Large parts of the year are reserved in advance for seasonal events, which absorb the best promotional space and the largest supplier contributions.
Ordinary lines are pushed into the quieter weeks between those blocks, which is when unglamorous staples receive their deepest cuts.
The weeks immediately after a major holiday tend to be strong for everyday goods, because the seasonal stock has cleared and space needs filling.
The calendar limits how retailers respond to costs
When supplier costs move suddenly, the promotional plan cannot easily be rewritten. Commitments have been made and print deadlines have passed.
Retailers adjust instead through the depth of the discount, the volume allocated, or by quietly reducing pack sizes on non-promoted lines.
What appears to be a swift reaction to conditions is usually the slow calendar meeting a faster world, with the gap closed by adjustments you can see on the shelf.