"À la rentrée ": see you in September/when we go back to school/after the holidays. Also means a set moment in time, that point in September when everything returns to normality.
I find it quite curious how the French seem to just take weeks off during August and migrate South to the coast of France pour les vacances. This just wouldn't happen in England! a) we don't have the weather and b) people wouldn't put up with turning up to "closed" shops when they desperately need something....!