All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
The engagement party or celebration is permissible if it is free from religious prohibitions, like mixing, women not wearing Hijab, and music, as we are not aware of any scholar who forbade it. However, the engagement party which people organize today whereby the fiancé meets his fiancée before conducting the marriage contract with her, and he puts a necklace or bracelet on her, then this party is forbidden and it is not permissible to approve of it, let alone participating in it. That is because before the fiancé conducts a marriage contract with his fiancée, it is not permissible for him to be in seclusion with her or shake hands with her, let alone seeing her in her complete beauty as it is the case in such parties.
As regards after the marriage contract, then she is his wife and they may organize a festival in order to announce this marriage; the Prophet said: "Make the marriage public." [Ahmad, At-Tabaraani, Al-Haakim and Ibn Hibbaan] However, this festival should be free from any prohibition, like mixing, songs accompanied with musical instruments and obscene language, and other prohibitions. Nonetheless, it is permissible for women to use the Duff (a tambourine-like instrument without bells) as the Prophet permitted it in marriage.
Allaah Knows best.