Using Jekyll's front matter tags, we want to be able to tag posts & events. This milestone consists of developing a tagging system that will enable visitors to sort by specific tags (tutorial, experimental, OpenStreetMap, cartography, etc.)

See Jekyll documentation on Front Matter and using Tags

It also looks like there is a complication about using tags with Github Pages. We'lll have to figure out how to get around this---seems to be an issue with Github not supporting Jekyll Plugins
Here's how someone solved this