Starting a business mainly aimed at supplying food and drink for wedding parties and Christian celebrations

It is haraam to start this business, because these celebrations that you mentioned in the question are haraam, either because of their origins or because of the evils involved in them. 

With regard to parties for Christian festivals, such as Christmas, New Year and other Christian celebrations, it is haraam for the Muslim to hold such celebrations or participate in them. This has been explained in fatwas no. 50074 and 175577. 

Therefore they are celebrations that are haraam because of their origins; moreover, they are not free of evils in the way they are celebrated. If something is haraam or involves prohibited actions, it is haraam to help others with it, because Allah, may He be exalted, says (interpretation of the meaning):

“Help you one another in Al‑Birr and At‑Taqwa (virtue, righteousness and piety); but do not help one another in sin and transgression. And fear Allah. Verily, Allah is Severe in punishment”

[al-Maa’idah 5:2].

With regard to Christian wedding parties, they are not haraam because of their origins per se, but they will never be free of prohibited matters such as free mixing, wanton display, singing, music and so on. 

So they come under the same rulings as the parties mentioned above. Therefore it is not allowed to take part in them, because what is required of the Muslim is to denounce these evils and not participate in them or approve of them. 

It was narrated that Abu Sa‘eed al-Khudri said: I heard the Messenger of Allah (blessings and peace of Allah be upon him) say: “Whoever among you sees an evil action, then let him change it with his hand [by taking action]; if he cannot, then with his tongue [by speaking out]; and if he cannot, then with his heart – and that is the weakest of faith.”

Narrated by Muslim (78). 

And Allah knows best.