Incorrect day of week:
If included, day-of-week must be the day implied by the date specification.
As an example, the following is an incorrect date:
<pubDate>Sat, 31 Dec 1999 23:59:59 EST</pubDate>
The following is a corrected example:
<pubDate>Fri, 31 Dec 1999 23:59:59 EST</pubDate>
This may be indicative of a more significant software error in the formatting of dates that needs to be corrected.
If it turns out that computing the correct day of week is impractical using the software you have available, then RFC 822 permits omitting both the day of the week and the subsequent comma from the value.
Not clear? Disagree?
You might be able to find help in one of these fine resources.