Quote:
4 does not equal 2.
... this might be more accurately described as the "first day of sale at the philatelic sales unit", but it is not a second day of issue
Yes, technically this is correct. But in general parlance, as a collector of these, I can search
ebay for "Second Day" and find what I am looking for. I expect that these will be listed as Second Day cancels, even if it is 3 or 4 calendar days after the official First Day ceremonies.
Like "Tax Day" is always April 15th, unless April 15th is on a Sunday, then Tax Day is April 16th. Or the "New Year's Day Parade" is held on January 2nd if the 1st is on a Sunday. We don't call it the "Day after New Year's Day Parade" that year, people still refer to it as the "New Year's Day" parade.
Here is an
ebay listing showing a specific category of Cancellation Type for "Second Day of Issue". I would expect to find the August 28th Washington DC covers listed like this, even though accurately it is the 4th day of issue. There is not a category for "first day of sale at the philatelic sales unit", we just use the shorthand "Second Day Cancel" as an umbrella.

Here are some of my "Second Day" DC cancels SOTN. Some of them may be 3rd or 4th day like the August 28th example, but I don't have a separate page for "Second Day cancels unless the First Day was on a Friday or Saturday".

</end of friendly rant>