Help!!!!!What to wear to a 40thy wedding Anniversary party.

willamsky

Im going to a 40th wedding anniversary party on the 29th of october.I live in canada so its a kind of cold around that time.And  the invitation said something formal, but i dont have the money to buy something new.I have a yellow maxi dress that can be dressy but i dont know what to wear with it.I have a black dress but i dont know how to wear that either.So i really need help.Please thanks to anyone who can.