challenge

 challenge  is an event-like post which serves as a (typically open) invitation for people to post achievements towards some goal, like running a total distance, or posting photos at some frequency (every day), supported by silos like Strava and micro.blog.

IndieWeb Gift Calendar
An annual group effort to gift (ship) one or more IndieWeb-related thing(s) each day of December that others can use to improve their IndieWeb experience. Started in 2017.

micro.blog

 * 2017-11-09 Micro.blog Photo Challenge (see also: https://www.manton.org/2017/11/5991.html)

Maxwell Joslyn
hosted a "Sith Lord Challenge" on his personal website at Sith Lord Challenge and collected responses via Webmention

Other Examples

 * 2019-01-03 Edublogger #blogging28 28 Day Challenge
 * #31daysiboc highlighting indigenous and teachers of color as writers and scholars.
 * https://www.strava.com/challenges/873
 * #100DaysofCode
 * National Blog Posting Month