An intercalary month is an additional month added to a calendar to synchronize it with the solar year or lunar cycles. This adjustment helps to keep seasonal events aligned with the calendar, preventing drift over time. Different cultures have used intercalary months, such as the ancient Roman calendar, which included an extra month called Mercedonius. In the Hebrew calendar, an intercalary month, known as Adar II, is added seven times in a 19-year cycle. This ensures that the lunar calendar remains in sync with the solar year, allowing important festivals, like Passover, to occur in their appropriate seasons.