Bukhari

Hadith Commentary:
If it is a voluntary (nafl) fast, then it is better to break it and participate in such invitations, because this increases mutual love and creates social harmony.

Hadith Commentary:
(1)
One should attend a wedding feast (walimah); however, it is not necessary to eat the food there. Thus, it is mentioned in the hadith that the Messenger of Allah sallallahu alayhi wa sallam said:
“When you are invited to a meal, accept the invitation; upon arriving, if you wish, eat, and if you wish, leave it.”
(Sahih Muslim, Kitab al-Nikah, Hadith: 3518 (1430))

(2)
If someone is observing a voluntary (nafl) fast, it is permissible to break it for the sake of accepting an invitation, because this increases love and fosters mutual relations.
Hafiz Ibn Hajar rahimahullah has transmitted a narration from al-Tabarani that a man arranged a meal and someone said:
“I am fasting.”
The Messenger of Allah sallallahu alayhi wa sallam said:
“Your brother has invited you and has made an effort in this regard; break your fast, and if you wish, fast another day in its place.”
(al-Mu‘jam al-Kabir by al-Tabarani: 152/4, no. 3264, Maktabat al-Ma‘arif edition)
However, its chain of narration is weak, but it can be presented as corroboration and supporting evidence.
(Fath al-Bari: 308/9)