Open the calendar you wish to edit by selecting it from the app's main screen. Tap on 'Edit' in the top right to switch into editing mode.
Use the left and right arrows on the top to move the calendar view forward or backward in time until the desired day is in view. Tap on the day where you want to add the event to highlight it. A yellow box will highlight your selection.
Tap on "Add Event" (or plus sign) to bring up the Add Event dialog. Select an image and adjust the date, time, duration, alarm and repeat fields as desired. Tap "Done" when finished.