Photosynthesis is the process by which plants use energy from the sun to make food. They use carbon dioxide from the air and water from the soil to make sugar and oxygen. Most plants release oxygen only during the day, when the sun can power photosynthesis. The exceptions to this general rule are the plants (mostly cacti, bromeliads, and certain succulents) that rely on an alternative photosynthetic pathway called crassulacean acid metabolism, or CAM, which allows them to keep their leaf stomata closed during the day to reduce water loss. These plants do release some oxygen at night when the stomata open and the oxygen can escape.
Carbon dioxide is not released during photosynthesis, but small amounts of that gas are emitted both day and night as a by-product of cellular respiration. It is worth noting that the majority of plants absorb carbon dioxide during the day for photosynthesis and do so in greater amounts than they release for cellular respiration.