Ask your venue or your outside catering company to consider seasonal, local, organic and fairtrade ingredients. Also source any wine, beer and other drinks from suppliers that are as local to your venue as possible. The more local, the smaller your carbon footprint. You could even consider serving a fully vegetarian or vegan meal. 

Ask your venue or caterer about their recycling policies and how they dispose of any rubbish. After the celebrations, arrange for any donatable leftovers to be delivered to a local food bank, homeless shelter or other well deserving organisation or charity. Be sure they will be happy to accept your generous offer beforehand though.