The phrase "to use a calendar" means to regularly look at and refer to a tool that helps you keep track of dates, events, or appointments. It helps you remember and plan things better in a well-organized manner. Full definition
Due to the nature of particular businesses, some companies do not use the calendar year for their bookkeeping. (hsbc.ca)
On the page of the trek, tour or package that you'd like to book, simply choose a departure date using the calendar on the left side of the page. (sastravelperu.com)