The Intercalary Days (or Ayyám-i-Há, meaning “Days of Há”) are days of preparation for the Fast; days of hospitality, enjoyment, charity, paying special attention to the plight of the poor and sick, and giving presents. They do not mark any particular event, but the way they are celebrated is, in some ways, similar to the way Christmas is observed worldwide. In the Gregorian calendar they usually fall from February 25th to March 1st depending on a Leap Year. In the Bahá’í Calendar they fall between the months of Mulk(Dominion) and ‘Alá’(Loftiness)
The "Ayyám-i-Há" are days to be spent in praise of God, in rejoicing, the giving of gifts, and acts of charity. Bahá'u'lláh directs His loved ones "with joy and exultation to hail and glorify their Lord, to sing His praise and magnify His Name" during them: The Most Holy Book, p. 25
Story about Ayyám-i-Há:
In the spring season Bahá’u’lláh used to stay at Mazra‘ih for some time.[ Bahá’u’lláh did not live at Mazra‘ih or Bahji all the time. He used to go and stay in ‘Akká sometimes] Mazra‘ih is situated at a distance of about two farsangs [about 12 kilometers] from the city of ‘Akká. To attain His presence I used to go to Mazra‘ih in the daytime and at night I stayed at the Pilgrim House.
On the first day of the Ayyám-i-Há [Intercalary days] one of the pilgrims had invited Bahá’u’lláh and all the believers in ‘Akká to lunch. I too went to Mazra‘ih. Early in the morning a large tent was pitched in front of the entrance to the garden on a delightful open space. That morning all the believers, numbering almost two hundred, consisting of those who were living in the Holy Land and the pilgrims, came to Mazra‘ih.
Around the time of noon, the Blessed Beauty came down from the Mansion and majestically entered the tent. All the believers were standing in front of the tent. Then Mirza Aqa Jan, standing in the presence of Bahá’u’lláh chanted a dawn prayer for fasting which had been revealed on that day. When the prayer was finished the Blessed Beauty instructed all to be seated. Every person sat down in the place where he was standing. His blessed Person spoke to us and after His utterances were ended He asked, ‘What happened to the Feast, is it really going to happen?’ Thereupon a few friends hurried away and soon lunch was brought in. They placed a low table in the middle of the tent. His blessed person and all the Aghsan[The male descendants of Bahá’u’lláh] sat around the table and since there was more room, He called some by name to join Him. Among these my name was called; He said, ‘Aqa Tahir, come and sit.’ So I went in and sat at the table in His presence.
At some point Bahá’u’lláh said, ‘We have become tired of eating. Those who have had enough may leave.’ I immediately arose and His blessed Person left. At first the food which was left over on His plate was divided among the friends, and then group after group entered the tent and had their meal. Everyone at this feast partook of both physical and spiritual food. (Adib Taherzadeh, The Revelation of Bahá’u’lláh, vol. 4, p. 8)