Relapsing fever, as its name implies, is an illness characterized primarily by recurrent episodes of fever, often accompanied by fatigue, malaise and other constitutional symptoms. It is caused by at least 15 spirochete species belonging to the genus Borrelia, and can be vectored to humans by either lice or ticks. Louse-borne relapsing fever (LBRF) is caused by Borrelia recurrentis and is transmitted from human to human by the body louse, Pediculus humanus humanus.
This means that the calendar has a cycle of 400 years as far as leap years are concerned. Every 400 years the same leap year schedule appears. During that cycle, 97 leap years occur (once every 4 years except 3 century changes, so 400 / 4 - 3 = 97). By pure coincidence, this happens to be just an integral number of weeks as well This means that every 400 years the Gregorian calendar repeats itself exactly on the same days of the week!
Calendar with 12 months. The Gregorian calendar is divided into 12 irregular months. Each month has either 28, 30, or 31 days during a common year, which has 365 days. During leap years, which occur nearly every 4 years, we add an extra (intercalary) day, Leap Day, on 29 February, making leap years 366 days long.