So it's only Thursday that is not showing? Everything else is showing?
See my screenshot below - I just added a session to MyPlanner on Thursday, but unless I scroll the date bar, Thursday barely looks visible. Can you verify that your Thursday sessions are not just off screen?
Also, Can you add a new session into any of Monday or Tuesday just as a test? Not necessarily something you will attend - you can delete it from your planner shortly. Does this new session eventually synch to your desktop?
--Margaret Ohayon, Esri