The app will only show you races that you can start to track on that day.
Once you join the race the app starts to use the GPS to collect position data so it would be a huge battery drain to join the race hours or days beforehand. Best practice is to start the tracking when you go to the boat and start to rig.
You can upload a GPX file of your tracks to Sailchart once you have finished sailing. However live tracking is only available if you use the app or dedicated GPS tracker.