Create a calendar item from an email on a different calendar by opening your calendar on another page.
Right-click the Calendar link from the Navigation pane. Choose Open in a New Window. Click on the title bar on your Mail window and drag it to the left-middle part of the screen until it snaps to occupy half the screen. Do the same to the calendar window, except drag it to the right-middle. Make the calendar you want to work with visible. Then, drag an email from the mail window to the correct date on the calendar in the calendar window.