Samedi the Deafness

One morning in the park James Sim discovers a man, crumpled on the ground, stabbed in the chest. In the man's last breath, he whispers his confession: Samedi.
What follows is a spellbinding game of cat and mouse as James is abducted, brought to an asylum, and seduced by a woman in yellow. Who is lying? What is Samedi? And what will happen on the seventh day?