The Question's Title (repeated in the body too) is :
Should UTC timestamps include day of the week or not?
The answer is "it depends":
- If humans don't need to know the Day-of-Week for a timestamp, then No.
- If humans do need to know the day-of-week for the timestamp, but they can have the date processed for them order to determine the day-of-week, then no, the base timestamp does not need it.
- If humans do need to know the day-of-week for the timestamp, and the process for computing that information would be manual, or time-consuming, then having it embedded in the timestamp would provide a much better user experience.
- If no humans are involved, there's no need to include the Day-Of-Week.
That's the basic answer to the core question.
The question divides the components of the feedback in to 'helpful' and 'harmful':
helpful: Allows the viewer to see, at a glance, what day of the week (UTC) the event occurred on
That's the whole idea. The user needs to know the day, so telling them at a glance is surely helpful.
harmful: Makes it more likely that a viewer interested in the day of the week will miss the fact that it's a UTC timestamp and will take the shown day of the week as valid
That's a bigger UX item than the focus the day-of-week only. There's already confusion about the UTC timestamps, and users who 'hover' over those timestamps are mostly (all?) already familiar with the fact that it is in UTC. Adding the Day-of-week will not make those users any more confused. It is not the DayOfWeek that makes the timezone confusing, but the UTC itself. Additionally there are other reasons why UTC is/was chosen, and those reasons are beyond the scope of this question/answer.
Bottom line, no, adding the day-of-week will not make it any more confusing than it already is. Many people miss that the timestamps are UTC already, and that's a different UX problem.
Applying that logic to the example provided in the meta.se question:
- there's a timestamp
- there's a human element
- there's a need to know the day-of-week for the timestamp
- the system does not allow an easy, or automatic ability to present the Day-Of-Week for the date.
For the situation described in the meta question, the right answer would be to include the Day-of-week in the timestamp.
An interesting User Experience question would be where in the timestamp, but that is not what this UX question asks.
The tail of the question is:
Is there UX literature that addresses this question, or are there standard UX design principles that do?
The basic question is: A user needs to know the Day Of Week for a date, the US Government has a website tailored for Usability: www.usability.gov and it has the following key components for usability:
image in public domain, but credited to Peter Morville
Where the 'Usable' aspect is defined as "Site must be easy to use", and the 'Findable' aspect is defined as "Content needs to be navigable and locatable onsite and offsite"
Making the required information (day of week for a date) only available 'off-site' is clearly a violation of the Usability and Findability aspects.