You say "sack dress", I say "yes please". I found this cotton rectangle in a pile at a church bazaar right across the Seine from where I'm staying. The shape transforms once on the body and visiting Warden Juliann Wilding obliged.
The label tells a tale of romance, tragedy and courage: Christian Aujard, born in Brittany in 1945, met his future wife Michele while working in the fashion industry. In 1968 they formed a ready-to-wear collection for women, later expanding into menswear. Though the late 60's and 70's the collections were celebrated for their innovative cuts, use of natural fabrics, and relaxed, yet chic look. Christian focussed on day wear and Michele created evening dresses, eventually designing under her own name. The tragedy of Christian's accidental death in 1977 did not see an end to his label. Michele continued both lines, designing soft flowing garments and innovating new colour palettes for men. The label continues today.
I'm guessing that this dress is from the early 80s. The way the fabric creates an inverted triangle evokes the silhouette of the 1980s while the relaxed drape, pattern and fabric choice are linked to the 70s.
Designer information from Fashion Encyclopedia.