You know that: You're looking forward to playing Volleyball or basketball or whatever, you come into the gymn and there is nobody there except you and the other guy who was stupid enough to think that there would be enough people there to play.
That is where a myriad of hacks like mailinglists, web-calendars, excel-lists etc. come in that try to keep track of who's coming in on a given trainingday and which lame sucker will stay put on the couch. And are you enough to play?
Trainingday helps you with that: It keeps track of your players and your trainingdays, sends out reminders and collects the responses: An hour before the training it sends out a report to all your teammembers with the aggregated responses: You will see with one glance if it makes sense to get up from the couch or if you should get up right now and play!