Because of this long hiatus from work and school, going back to school in the fall is a big deal that carries more weight than here in the States. The first day back is called "la rentrée," which translates literally to "the reentrance."
On a deeper level, à la rentrée is much more than a calendar date. It's about moving forward with new focus come September. It's time to pull your big girl boots on, roll up your sleeves and get serious.
Playtime is over. It's time to leave long lazy summer days behind and reenter reality.
Viva la rentrée!