Like the other topics this quarter, "Pigs might fly" is an animal-related idiom. If you enter this week, you don't have to write about literal flying pigs. The meaning of the idiom is something along the lines of
That [named circumstance] will never happen
That [named circumstance] is impossible
"Pigs might fly" is usually said sarcastically. It might go something like this:
Jan said, "I might clean out the entire basement this weekend."
"Sure," Ben replied. "And pigs might fly."
So while it would be permissible to write an entry about flying pigs, please be aware that many other people may write something similar, and that the judges may quickly grow weary of reading such entries. It's possible to write "on topic" for this week without mentioning pigs at all, if you write an entry that incorporates the meaning of the idiom.