The 'Trim Calendar Items at' option allows you to limit the number of characters that are shown in the calendar.
Click on the drop-down box to view the options listed for 'Trim Calendar Items. These numbers signify the number of alphanumeric characters that can be displayed on the particular day in the calendar. The choices go from 'Don't trim calendar items' to 8, 10, 12, 15, and continue in sequences of 5 all the way to 50. Depending on the choice you make, that is the number of characters that will appear in the title of the day of that event on 'My calendar'.