The Kitáb-i-Aqdas - The Most Holy Book
Index term: Hospitality
k16.
O Pen of the Most High! Say: O people of the world! We have enjoined upon you fasting during a brief period, and at its close have designated for you Naw-Rúz as a feast. Thus hath the Day-Star of Utterance shone forth above the horizon of the Book as decreed by Him Who is the Lord of the beginning and the end. Let the days in excess of the months be placed before the month of fasting. We have ordained that these, amid all nights and days, shall be the manifestations of the letter Há, and thus they have not been bounded by the limits of the year and its months. It behoveth the people of Bahá, throughout these days, to provide good cheer for themselves, their kindred and, beyond them, the poor and needy, and with joy and exultation to hail and glorify their Lord, to sing His praise and magnify His Name; and when they end -- these days of giving that precede the season of restraint -- let them enter upon the Fast. Thus hath it been ordained by Him Who is the Lord of all mankind. The traveller, the ailing, those who are with child or giving suck, are not bound by the Fast; they have been exempted by God as a token of His grace. He, verily, is the Almighty, the Most Generous.
k57.
Verily, it is enjoined upon you to offer a feast, once in every month, though only water be served; for God hath purposed to bind hearts together, albeit through both earthly and heavenly means.
n29.
these days of giving that precede the season of restraint
Bahá'u'lláh enjoined upon His followers to devote these days to feasting, rejoicing and charity. In a letter written on Shoghi Effendi's behalf it is explained that "the intercalary days are specially set aside for hospitality, the giving of gifts, etc.".
n82.
Verily, it is enjoined upon you to offer a feast, once in every month
This injunction has become the basis for the holding of monthly Bahá'í festivities and as such constitutes the ordination of the Nineteen Day Feast. In the Arabic Bayán the Báb called upon His followers to gather together once every nineteen days to show hospitality and fellowship. Bahá'u'lláh here confirms this and notes the unifying role of such occasions.
'Abdu'l-Bahá and Shoghi Effendi after Him have gradually unfolded the institutional significance of this injunction. 'Abdu'l-Bahá emphasized the importance of the spiritual and devotional character of these gatherings. Shoghi Effendi, besides further elaborating the devotional and social aspects of the Feast, has developed the administrative element of such gatherings and, in systematically instituting the Feast, has provided for a period of consultation on the affairs of the Bahá'í community, including the sharing of news and messages.
In answer to a question as to whether this injunction is obligatory, Bahá'u'lláh stated it was not (Q&A 48). Shoghi Effendi in a letter written on his behalf further comments:

Attendance at Nineteen Day Feasts is not obligatory but very important, and every believer should consider it a duty and privilege to be present on such occasions.