Doubt about this situation with the particle は


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.

