The months follow a lunar cycle, so each month is 28 days long. The months also seem to follow a solar cycle as well, as the Rokugani year always begins on the first day of winter (Please see my page on the Festivals of Rokugan for an explanation of why I go against the written canon here). The sources are unclear if the year in Rokugan is exactly 336 days long, or if there are "leap days" to let the appropriate months match the appropriate solar phenomena (solstices and equinoxes).