Calendar is an alternative view of your notes.
Month view is the default one when you click on a Calendar link in top navigation bar. Current day is marked red. When a date has an orange bar on the left, it means that there are notes which will expire that day. When it has blue bar, at least one note was created that day. You can click on any date to enter day view and inspect them.
Both notes created and expiring on given day are shown in this view.