Garmin Express does not have a calendar. Are you referring to the calendar in Garmin Connect? If so, this is not an Express question but a Connect one.
I found the following FAQ which may or may not assist you. If not, it would be best to post this on the Garmin Connect forum or call Product Support for assistance.
How do I create an advanced workout in Garmin Connect?
Thanks Lisa. You are right, I was asking about Garmin Connect.
Unfortunately the workouts page doesn't allow me to upload a training plan from a file. Although I could manually create each workout. I'm beginning to think the upload cant be done.
The only way to get custom workouts to your 920 is by Garmin Connect or the obsolescent Garmin Training Centre (still out there somewhere). I think you might be able to get them into GTC but not sure as it's been such a long time since I've used it.