Fireflies

I went to the Fireflies Festival on Saturday at Nanase River Park. We could see a few fireflies which might be released just before opening it. We went their last year and saw a few fireflies, the festival had already finished. So we thought the best day had already gone at that time. It might be the best day to hold a festival, but there were a few fireflies. I supporse that fireflies are now decreasing because of getting dirty of river. I think, there were a lot of fireflies when the festival had been established. The earth is getting dirty.

* If there's any native english speaker who could read this diary, could you please correct my grammer? I will erase these sentences after the comment to correct my english.