1. Last night, we cooked our dinner over a fire. (meaning: yesterday night) 2. On the last night, we cooked our dinner over a fire. (meaning: the final night of a sequence of nights) Example: On the first night, we checked into our hotel. On the second night, we enjoyed a dinner cruise. On the third night,... On the last night of our wonderful vacation, we cooked our dinner over a fire and it was so romantic.

It should be our dinner not or. When they say on the last night its a time marker meaning it was something they did during their last night. It's kind of confusing because on can also mean something being on top of something else. Hope that helps.