The EventPilot App Builder displays tabs per day for your sessions. The days tabs shown are defined via your Settings > Basic Settings start and end date of your conference.
If you change the dates, you are hiding the tabs in the Agenda page. You can use the list tab in the agenda to see the full list of sessions you have added, even if those sessions are outside your event start and end dates. From there, you can delete delete or edit those sessions.
For example, if your original event dates were September 17-19 and you change the dates in Basic Settings to September 18-19, the agenda table displays tabs only for September 18 and September 19. You can still see any sessions for September 17 when switching to the full list view.
Note: all sessions in your agenda data appear in the EventPilot Conference Apps.