If the wedding party has no objectionable aspects, such as mixing between men and women, shameless singing, or if the person attending has the ability to put an end to those aspects, it is then allowed for her to attend such a gathering to participate with the others in the happiness. In fact, it is a must to attend if the person has the ability to remove the objectionable aspects.
If the party, though has objectionable aspects and the person does not have the ability to stop them, then she is not allowed to attend such a party. This is based on the generality of Allah’s statement ,
'' And leave alone those who take their religion as play and amusement, and are deceived by the life of this world. But remind [them] with it [the Qur’an], lest a person be given up to destruction for that which he has earned, when he will not find for himself any protector or intercessor besides Allah.” [al- Anam: 70]
Allah also says,
''And of mankind is he who purchases idle talks [singing, music] to mislead (men) from the Path of Allah without knowledge, and takes it by way of mockery. For such their will be a humiliating torment.” [ Luqman:6]
The hadith that has been narrated condemning singing and musical instruments are numerous.
As for birthday parties, neither a Muslim man or a Muslim women should attend them because they are innovations. The only exception would be to attend them to put a stop to them and explain the rule concerning such parties.