The Weekly Challenge starts with a topic announcement sometime over the weekend. Photos are submitted for 7 days starting Wednesday (00:00 UTC). The photos are "judged" by the members who add their likes to each photo. The top liked photos are then added to a poll that runs for 3 days while members can cast one vote for one photo. The photo with the most votes is declared the winner.
Find out more here: Weekly Photo Challenge - Rules and Guidelines