I think you have two problems:
1. You haven't used は to contrast anything. It is just used as a typical topic marker in the two sentences, with the topic changing from May to August.
2. Your adjectives need to be conjugated, or you need to use different verbs.
Does that give you a hint or two?
Edit: Actually, I'm trying to come up with a sentence that uses は as a contrastive marker for the example you gave, and I think any example will end up being a bit complicated.