What had the Wargs and the Goblins planned for that night? In other words, why were the Wargs and Goblins meeting in the forest that night to begin with?

The Wargs were meeting the Goblins to conduct a raid.

The Wargs and the goblins often helped one another in wicked deeds. Goblins do not usually venture very far from their mountains, unless they are driven out and are looking for new homes, or are marching to war (which I am glad to say has not happened for a long while). But in those days they sometimes used to go on raids, especially to get food or slaves to work for them. Then they often got the Wargs to help and shared the plunder with them. Sometimes they rode on wolves like men do on horses. Now it seemed that a great goblin-raid had been planned for that very night. The Wargs had come to meet the goblins and the goblins were late.