What is "ayyam-i-ha"

Ayyám-i-Há refers to a period of intercalary days in the Bahá'í calendar, when Bahá'ís celebrate the Festival of Ayyám-i-Há. The four or five days of this period are inserted between the last two months of the calendar.

The number of days between the 18th and 19th months (Mulk and `Alá'), and therefore the length of Ayyám-i-Há, vary according to the timing of the following vernal equinox. so that the next year always starts on the day of the vernal equinox. Prior to 172 B.E. (2015 A.D.), Ayyám-i-Há was from sunset on February 25 to sunset on March 1 which caused the calendar to be automatically synchronized with Gregorian leap years.