ATG is simply an abbreviation of 'Artist's Training Ground', which is where a series of prompts will be posted for an artist to create a totally new image for in the allowed time. EQD have normally run one of these events for 30 days around September time for the past few years. It's always best to look out though, as they could start at any time.There are however, other pony ATG options available. One that's worth considering is MLP-ATG-Alumni who post weekly prompts for people to try out.