The field Description in the sitecontent type Event (used in the calendar app) is defined in the site column group _Hidden. The type of the text is : RTF- text (middle option). In a calendar list it is not possible to change the type of the text. One can imagine that because this field is can be synchronized with Outlook.
I’ve used this field in NewForm and EditForm and both forms contain tabs.
If I open the EditForm or change a tab the value in this field is erased and the synchronisation with Outlook is lost.
Saving the form without changing tabs does save the text correctly and restores the Outlook synchronisation.
Unfortunately the SP 2007 style rich text editor is not compatible with DFFS when using tabs. The only way you can try making it work with tabs is to place it at the top OR at the bottom of the tab, but if you don’t use tabs it should work.