Not sure if this useful for others but I've done a list of the various weekly and monthly challenges I have found. It's not intended to be exhaustive. And I won't do all of them each week
But others may find it useful
docs.google.com
But others may find it useful
Photography challenges
Photography challenges Photography challenges Due date (time listed under challenge as well as whether GMT, UTC etc),Challenges <a href="https://www.flickr.com/groups/macromonday/discuss/">MACRO MONDAYS | Flickr</a>,<a href="https://www.flickr.com/groups/crazy_tuesday/discuss/">Crazy Tuesday | F...