I have never noticed this but I have a few ideas to explain this so called phenomena:
1. You bought your crisps on a saturday and the food safety comity have estimated that the safest period of time to eat your crisps is within 7 days.
2. Regarding the above the crisp suppliers may only package their crisps on saturdays as they spend all of Monday, Tuesday, Wednesday, Thursday and Friday producing the crisps themselves.
3. You may have been viewing a select number of crisp packets all from the same batch which you bought all from the same supermarket.
4. As Friday is the day of "Frying", crisps are at their all time best on the day they are made so Saturday is the day of decay.
5. Perhaps Mr. Walker of Walker's crisps was in fact born on a saturday (or maybe even passed away on a Saturday as that may be more fitting to the context) and therefore the date remains significant in the crisp world.
6. The potato-particals which make up crisps have date-sensitive cells which react only on saturdays
7. Food products of the carbohydrate variety degenerate precisely 24 hours before the day of rest as all carbohydrates are Christian.
I also have a hypothesis for why the Americans have the name for "crisps" wrong:
In England "chips" are known as "crisps" as 'fries' are called "chips". When transporting the first batch of "crisps" to America there was a mistake in the naming as the crisps in fact arrived on a Sunday, by which time of course they had lost their flavour, making them taste curiously like chips. So for this reason the Americans renamed them chips and had to steal the French word for chips to differentiate between the two- thus contriving in "fries".