Tonight the earth will experience a blue moon. A blue moon is called by that name not because it is blue, because it is rare--two full moons in a single month. The next chance to see one will arise in 2018. This astronomical event leaves several possibilities for free writes:
(1) Write about something rare.
(2) Write about an unusual image from your week.
(3) Let something be understood out of context. Use that misunderstanding as the jumping off point for your writing.