On genki 1, on the page 107 there are the following phrases: あしたは日本語のクラスがありません。 あした東京でお祭りがあります。 Why the は particle is used on the first one and not in the second?
The は in the first sentence is contrastive. It suggests that tomorrow, in particular, there's no class. It could be that tomorrow is a holiday, or in response to someone mentioning tomorrow's class without realizing that tomorrow is Saturday.
The second sentence, without the は, is neutral. It simply says there's a festival in Tokyo tomorrow.
I don't have my Genki I handy, but there's a sidebar somewhere in there about how contrastive は is very common in negative sentences--probably because you're not likely to talk about the absence of something unless someone is expecting it to be present, which is a contrastive situation.