COUNT (Distinct Column) = 0 With GROUP BY

I have a table of users and date when they logged on to a system. I am trying to count how many distinct users logged on for each day of the week. The SQL below works when there's at least a user for each day. But when there is no user for a particular day such as Sunday, I still want it to return "SUN



0 "

I learned that you can use GROUP BY ALL and it works but the "ALL" is deprecated beyond SQL 2005.

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SELECT UPPER(LEFT(DATENAME(dw, StartTime), 3)) AS DayOfWeek,
COUNT(DISTINCT UserID) AS NumberOfUser

FROM testUserLoginDuration
WHERE Archived = 0
GROUP BY UPPER(LEFT(DATENAME(dw, StartTime), 3))
ORDER BY
CASE WHEN UPPER(LEFT(DATENAME(dw, StartTime), 3)) = 'MON' THEN 1
WHEN UPPER(LEFT(DATENAME(dw, StartTime), 3)) = 'TUE' THEN 2
WHEN UPPER(LEFT(DATENAME(dw, StartTime), 3)) = 'WED' THEN 3
WHEN UPPER(LEFT(DATENAME(dw, StartTime), 3)) = 'THU' THEN 4
WHEN UPPER(LEFT(DATENAME(dw, StartTime), 3)) = 'FRI' THEN 5
WHEN UPPER(LEFT(DATENAME(dw, StartTime), 3)) = 'SAT' THEN 6
WHEN UPPER(LEFT(DATENAME(dw, StartTime), 3)) = 'SUN' THEN 7
END

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returns

MON 6
TUE 3
WED 5
THU 3
FRI 2
SAT 1