Tanabata festival ius held either on the 7th of July or the 7th of August, depending on the area.
People write their wishes on small pieces of colored paper called Tanzaku, and hang them on bamboo trees put up in public places. Often, decorative lanterns too are hung on the trees. The bamboo and decorations are either burnt or sent floating on a river, after the festival.
Some of the wishes I could read (with my little knowledge of Japanese) were:
"To be able to eat delicious food that cannot be eaten everyday"
"I love Ryuu-chan - Minami " (Minami is the person who wrote the wish)
"I want my kids to study well"
"I wish to meet a better person than my boyfriend!"
"To have an enjoyable life without quarrel"
Most of the wishers write their given name too. However, it is unlikely that the writer can be recognized using only what is written in Tanzaku.
It seems tat tanabata provides a way to tell your wishes to the world. I think it is one way to have a public secret too, for those who feel like having one.