What is the ruling on working in the organization of mixed wedding parties that include dancing and singing, and is it permissible to perform the Hajj with money earned from this work?

Your work in this field is forbidden (haram (definition)) because it constitutes cooperation in sin, and the wage you receive is illicit (suht). You must abandon this disobedience and seek a permissible (halal (definition)) job. If Allah knows of your sincerity, He will open doors of lawful sustenance for you. Allah Almighty says: "If Allah knows any good in your hearts, He will give you better than what was taken from you." (Qur’an 8:70). As for your Hajj performed with this money, it is considered valid, though sinful, according to the majority of scholars. However, Imam Ahmad held the view that it is neither sound nor valid.