The Calvin cycle takes place in the stroma of the chloroplast. The stroma is the fluid-filled section of the chloroplast in which the thylakoids stack up in structures known as grana.
During photosynthesis, two major phases of reactions happen. The first phase is light-dependent and takes place mostly within the thylakoids. It utilizes light energy to produce energy molecules such as Adenosine Triphosphate (ATP). But the second phase is light independent and occurs in the fluid sections surrounding the thylakoids, called the stroma. This second phase involves the fixation and reduction of carbon dioxide. All the reactions in this second phase are collectively called the Calvin cycle.