Note that Monthly and Weekly challenges run in July, too.
This is an annual spinning challenge running concurrently with the Tour de France. In 2018 this is July 7th - July 29th.
What's the challenge? Spin yarn.
Where, how to take part: Challenge yourself and have fun! One site with information is www.spinfoolish.com/tour-de-fleece/
This is a drawing challenge running from July 1st to July 31st
What's the challenge? Draw animals from a prompt list that has one per day.
Where, how to take part: Find the prompt list (2018 at FB, 2018 at Twitter), draw, post to FB or anywhere using the tag "zoolyartchallenge".
This is a writing challenge running annually in April and July.
What's the challenge? Set yourself a wordcount goal and meet it.
Where, how to take part: Sign up on the official website, campnanowrimo.org
This is an art challenge running every year from June 1st to August 31st. You can take part in one, two, or three months.
What? Draw something every day to a topic you chose yourself.
Where, how to take part: Sign up at Livejournal or Dreamwidth. There is a Tumblr, too.
This is a drawing challenge running from July 1st to July 31st
What's the challenge? Draw werewolves
Where, how to take part: Post anywhere with the tag "julycanthropy"
This is a gamified art challenge running from July 1st to August 1st
What? Participants are split into two teams. You score points by drawing characters belonging to a member of the other team. The team with the highest score wins
Where, how to take part: Sign up on the official website, artfight.net