From Online Manual
Viewing the calendar
The Calendar that can be found on the Main menu of a SMF powered forum is used for showing holidays and events posted by members. The calendar is presented in the typical calendar fashion. If the first day of your week is not Sunday, you can adjust this in your profile.
At the top of the calendar you will see the current month and year. Underneath that you will see the seven days of the week. Located under that are the days of the month. If an event is posted on a particular day, you will see the event in that day's box. The same goes for holidays. At the bottom you will see links to go back one prior month and to go forward one upcoming month. You can also use the drop-down boxes in the middle to scroll to a particular month and year so you can view events and holidays for that particular month of that partiulcar year. Next to that you will see a link that says Post Event. This will lead you to a page where you can post a calendar event, assuming you have the permission to post events to the calendar.
Posting an event to the calendar
If the calendar is enabled on a forum and you are given the permission to post calendar events you will be able to Post an Event to the calendar by selecting Calendar from the Main menu followed by Post Event from the calendar screen. If the option is enabled you may also be able to simply click on the day of the month to post a new event on that day. What you see now is a Start new Topic screen with supplementary fields for the Event title, date of event and the board the event will have a topic posted to.
In addition, if you have the proper permissions, you may see a 'Link to Calendar' link at the bottom of each topic. Clicking this link allows you to Post a new Event in the calendar linked to the topic you were viewing when you you clicked on the 'Link to Calendar' link.