The Calendar view (http://www.insanejournal.com/users/exampleusername/calendar/) contained in every user's journal is not meant to keep track of personal schedules like other online calendar services. Rather, it serves as a record of your previous entries. As you post in your journal, each one is archived on your Calendar, according to the date that it is posted, for ease in locating later.
If you have posted on a particular day, the number of posts that you have made in that day will display in the date box, including private or protected posts. The number that displays in the date box will link to a Day page that displays all of the entries you made on that day. However, all of the usual security settings still apply (See http://insanejournal.com/support/faqbrowse.bml?faqid=63), and anyone visiting a Day page on your journal will see only entries that they are normally permitted to view.
Another feature of the Calendar view is the ability to view a list of all the posts, including subjects and number of received comments, that you've made in a given month, arranged by date. The month view is located here: http://www.insanejournal.com/view/?type=month&user=exampleusername . It is also linked from each of the month blocks on the Calendar page. If you are looking for a particular entry, this is often the best way to locate it, particularly if you remember the subject you used for that entry.