We planned to tag the challenges with the date when people are working on it during the OSCEdays. For example challenge XX is tagged June11-06-2015. When I click on the tag I can see all the challenges that are active on the 11th of June.
I think it might be relevant and less messy to put it as a category. Tags will be only used to describe the type of challenges and the task. Eg. Biogas, Software/Coding etc.
What do you think?