We are going to start working on this soon. Right now it will be one way syncronization from TrainerDay to Intervals. This will allow you to have the same training calendar every where but you must make changes to your TD calendar for them to appear every where else.
We are considering a limited/simple two way syncronization in the near future. You will only be able to add workouts via TD but other changes, like workout days and names and descriptions could be updated in intervals and syncronize. I think that will make intervals users the happiest other than full two way sync which we can’t do.