Crossing over occurs during Prophase I. Synapsis is the term used to describe homologous pairs group together after the chromosomes condense. Homologous pairs form close associations called chiasmata; it is in these areas where crossing over, the exchange of genetic material, takes place.
Source: Holtzclaw, Fred, and Theresa Holtzclaw. AP Test Prep Series. San Francisco: Pearson Education Inc., 2013. 81-82. Print., http://online.santarosa.edu/homepage/cgalt/BIO10-Stuff/Ch06-Cellular_Respiration/Electron-Transport-Chain-CPG-Notes.JPG