Bug 2976

https://tools.ietf.org/html/rfc5322#section-3.3 specifically states that day-of-week, and seconds for time-of-day, are optional.

No information is lost by omitting the day-of-week since it can be recovered from the actual date. Implementing a (hidden) preference to not include day-of-week in the Date: header can thus decrease the size of the message (slightly, but in terms of billions of messages sent it means significant savings).

Having the timestamp to accuracy of seconds also does not give significantly more information, especially since most users' clocks are not second accurate. Having a hidden setting for this can also mean significant storage/bandwidth savings.