What reaction begins the Calvin cycle?

Before the Calvin cycle begins the energy is stored in bonds of ATP

The Calvin cycle is the set of chemical reactions that take place in the chloroplasts during photosynthesis.

This cycle begins when a 5-carbon compound called ribulose 1-5-bisphosphate "catches" one CO2 molecule and makes a 6-compound molecule.

That's the start ... immediately after that the enzyme Rubisco breaks up the 6-compound molecule into 2 equal parts (2 molecules of 3-phosphoglycerate)