The activity orgazized by Wenqing is to sing an English song for

 

How about:

The activity orgnaized by Wenqing is to sing an English song for celebrating Mid-Autumn Festival.

The topic of singing an English song organized by Wenqing is Mid-Autumn Festival.

 

topic [ˈtɒpɪk]

n

1. a subject or theme of a speech, essay, book, etc.

2. a subject of conversation; item of discussion

3. (Philosophy / Logic) (in rhetoric, logic, etc.) a category or class of arguments or ideas which may be drawn on to furnish proofs