To Baha’is, the days named “Ayyam-i-Ha” mentioned in this prayer are the “Intercalary Days” established by Baha’u’llah and which appear in His Most Holy Book, the Kitab-i-Aqdas.
These Intercalary Days, or “Days of Ha,” occur between the eighteenth and nineteenth months of the Baha’i Calendar (February 26 to March 1 inclusive), and end one day before the Baha’i fast begins. There are four Intercalary Days in ordinary and five in leap years. These are days of preparation for the Fast; days of hospitality, charity, ministering to the poor and sick, the giving of presents, and so on.
In order to understand how the “Days of Ha” fit in the Baha’i calendar, it is necessary to understand this calendar.
It was the Bab, the precursor to Baha’u’llah, who introduced a new calendar which was later named the Badi' or Baha'i calendar. According to this calendar, a day is the period from sunset to sunset. The Bab ordained the month of 'Ala' (March) to be the month of fasting and decreed that the day of Naw-Ruz (New Year), which is on March 21st, should mark the termination of that period.
Naw-Ruz, therefore, is the first day of the Baha’i New Year. It coincides with the spring equinox in the northern hemisphere, which usually occurs on 21 March. Baha'u'llah explains that this occasion is to be celebrated on whatever day the sun passes into the constellation of Aries (i.e. the vernal equinox). The Naw-Ruz Festival could fall on 20, 21, or 22 March, depending on the time of the equinox.
The Badi' calendar is based on the solar year of 365 days, 5 hours, and 50 odd minutes. The year consists of 19 months of 19 days each (i.e. 361 days), with the addition of four extra days (five in a leap year) between the eighteenth and nineteenth months in order to adjust the calendar to the solar year. These four extra days are the Baha’i Intercalary Days. The Bab did not specifically define the place for the intercalary days in the new calendar. But Baha’u’llah resolved this question in the Kitab-i-Aqdas by assigning the "excess" days a fixed position in the calendar immediately preceding the month of 'Ala,' the period of fasting.
The Intercalary Days have the distinction of being associated with the letter “Ha." The abjad numerical value of this Arabic letter is five, which corresponds to the potential number of intercalary days. The letter "Ha" has been given several spiritual meanings in the Holy Writings, among which is as a symbol of the Essence of God.
Baha'u'llah enjoined upon His followers to devote these days to feasting, rejoicing and charity. Shoghi Effendi, the Guardian of the Faith, explained that "the intercalary days are specially set aside for hospitality, the giving of gifts, etc."