(CNN) -- Mark your calendars! On Saturday, September 22 -- the first day of fall in the northern hemisphere and the start of spring in the southern hemisphere -- we invite you to take part in CNN iReport's first one-day mobile photography challenge.
Your assignment is simple: On Saturday, use your mobile device to take a photo that illustrates your outdoor surroundings. The photo can be a landscape, skyline, street or action shot -- just make sure it is shot horizontally and is unfiltered.
Tell us what kind of phone you used to take the photo, time you shot it, as well as what the weather was like that moment. If you share it on social media, use the hashtag #hellofall or #hellospring!
The best shots will be showcased on CNN as part of "Our Mobile Society," an initiative examining the impact of mobile technology on our lives.