When viewing RSS feeds in Mozilla Thunderbird, you have the option of loading the actual (X)HTML version of the article/post/whathaveyou or simply the headline and summary. However, what loads fine(sent as XML) does not load fine in Thunderbird.
It turns out that named entities such as
are not recognized, whereas the numerical equivalent
To get my feed rendering fixed in Thunderbird I had to edit the get_calendar() function and a few other named entities custom to my template.
I think the ideal solution is for WordPress to simply convert to all numerical entity references.