Shinto's central ritual concern is not sin in the sense of moral transgression but kegare, a state of pollution or defilement brought on by contact with death, serious illness, blood, or other forces treated as disruptive to the vital order the kami preside over. Saikai names the deliberate discipline of avoiding, in advance, anything likely to bring on kegare in the days before a rite: eating meat, drinking alcohol, attending funerals, entering a house where death or birth has recently occurred, and in some traditional formulations even avoiding certain kinds of ordinary speech or contact considered coarse. The logic is preventative rather than remedial, distinct from misogi and harae, which cleanse a pollution already incurred; saikai instead tries to ensure the celebrant arrives at the rite already in a state close to purity, so that the purification performed immediately before the ceremony has less to undo.
The Engishiki, a compilation of Heian period ritual law completed in 927 CE, formalizes fixed periods of saikai, sometimes a single day and sometimes considerably longer, required of officiants before major state ceremonies, treating the discipline as a legal as well as a religious obligation for anyone serving the imperial cult.