You may be connected to more than one team and want to view all your games or you may want to sync your personal calendar to another service such as iCal (see calendar syncing for more details). Your personal calendar is only visible by you.
If using TeamPages classic, it will look like this:
Every event you're connected to will be automatically be placed on your personal calendar.
Now you know: If you're a team manager, be sure that you post events it in your Team Calendar so everyone can see them. Anything in My Calendar is only visible by you.