In fact, in the EU that is regulated by article 6A (see eur-lex.europa.eu/legal-content/EN/TXT/HTML/?uri=… ) . That states amongst other things that:

Any announcement of a price reduction shall indicate the prior price applied by the trader for a determined period of time prior to the application of the price reduction. … The prior price means the lowest price applied by the trader during a period of time not shorter than 30 days prior to the application of the price reduction.

So maybe it wouldn’t apply for a 3 month period, but it does prevent stores from increasing the price by say 20% overnight and then give a “10% discount” the next day.