What are the rules during sales periods in France?

In France, there are two sales periods in a year, one during the summer and one in the winter. These sale periods have fixed dates that are set every year. There are some specific rules during these periods, one of which is that goods' prices can only be reduced below their peak price during these sales periods. Setting fixed sales periods is bound to drive up sales during these periods, so there are some additional rules that need to be followed.