The Bahá'í calendar has 19 months of 19 days each. Let's do the math... that equals to 361 days. So, what happens to the other 4-5 days (if leap year) remaining? Bahá'u'lláh has given those days the name of The Days of Ayyám-i-Há (intercalary days) which precede the Baha'i month of 'Ala which is the very last month of the year in which Baha'is also observe the fast for 19 days.
Ayyam-i-Ha is a celebration in which Baha'is all over the world to charitable acts, rejoicing, visiting friends and the giving of gifts. It is an even happier time for children to whom gifts are mostly given to.
We want to share this beautiful prayer specially dedicated to the Intercalary days:
My God, my Fire and my Light! The days which Thou hast named the Ayyam-i-Ha  in Thy Book have begun, O Thou Who art the King of names, and the fast which Thy most exalted Pen hath enjoined unto all who are in the kingdom of Thy creation to observe is approaching. I entreat Thee, O my Lord, by these days and by all such as have during that period clung to the cord of Thy commandments, and laid hold on the handle of Thy precepts, to grant that unto every soul may be assigned a place within the precincts of Thy court, and a seat at the revelation of the splendors of the light of Thy countenance.
These, O my Lord, are Thy servants whom no corrupt inclination hath kept back from what Thou didst send down in Thy Book. They have bowed themselves before Thy Cause, and received Thy Book with such resolve as is born of Thee, and observed what Thou hadst prescribed unto them, and chosen to follow that which had been sent down by Thee.
Thou seest, O my Lord, how they have recognized and confessed whatsoever Thou hast revealed in Thy Scriptures. Give them to drink, O my Lord, from the hands of Thy graciousness the waters of Thine eternity. Write down, then, for them the recompense ordained for him that hath immersed himself in the ocean of Thy presence, and attained unto the choice wine of Thy meeting.
I implore Thee, O Thou the King of kings and the Pitier of the downtrodden, to ordain for them the good of this world and of the world to come. Write down for them, moreover, what none of Thy creatures hath discovered, and number them with those who have circled round Thee, and who move about Thy throne in every world of Thy worlds.
Thou, truly, art the Almighty, the All-Knowing, the All-Informed.
[1 The Days of Ha, Intercalary Days.]. (Compilations, Baha'i Prayers, p. 236)