What should I do about not fulfilling my promise to my friends to hold a success party for them, and is expiation due for that?

There are several scenarios for a promise:

1. If, at the time of the promise, there is no intention of breaking it, and no harm results to the promisee: fulfilling it is recommended by the majority of scholars and obligatory according to some.

2. If, at the time of the promise, there is an intention to fulfill it, and the promisee has undertaken an obligation due to the promise: fulfilling it is recommended by the majority of scholars and obligatory according to the Malikis.

3. If, at the time of the promise, there is an intention not to fulfill it: this is the hypocrisy mentioned in the hadith (definition): "When he promises, he breaks it."

Scholars unanimously agree on the recommendation of fulfilling a promise unless it is forbidden, but they differ on its obligation. Imam Shafi'i, Abu Hanifa, and the majority of scholars are of the opinion that it is recommended, while Umar ibn Abd al-Aziz and a group of scholars consider it obligatory. Breaking a promise with the intention of not fulfilling it is hypocrisy, and repentance for it is necessary. There is no expiation for breaking a promise except repentance, and it is advisable to mend the feelings of those whose promises were broken. It is also important to note the prohibition of intermingling between non-mahrams.