Precepts are 8 vows for one day only. This is traditionally done on new and full moon days and was a practice given to lay practitioners so that they could generate tremendous positive action. They also include observing a partial fast, eating one meal only at lunchtime and drinking only after lunch. Breakfast is optional. For more information, visit: http://buddhism.kalachakranet.org/resources/8_mahayana_precepts.html