In the Sumerian calendar, it was added every few years after one of the half-years to equalize the lunar and solar cycles. In the ancient Persian calendar, it was added once every six years to compensate for the difference with the tropical year. In the Jewish calendar, it is inserted before Adar (the twelfth month according to the traditional account or the sixth according to the modern one). In the Chinese calendar, it may be inserted after various months. In the accounting of banking institutions - a conditional month temporarily assigned to payment documents that actually arrived in the new year, but are subject to inclusion in the accounting for the previous year's entries. The need for this accounting technique has declined sharply with the transition to electronic payments.
Names of the thirteenth month:
In the Zoroastrian calendar - spandarmad-vihezag - "plug-in spandarmad." Mercedonium in the ancient Roman Republican calendar.