This message will usually come from the use of the EncodeJewishDate macro function.
In the Jewish calendar, on leap years, the month of Adar ir replaced by 2 months named Adar I and Adar II. On non-leap years, these 2 months do not exist, while on leap years, the month of Adar does not exist.
Topic 108368, last updated on 08-Mar-2019