T-SQL simplify repeated query unions for each day of the week
Syntax Issue

The derived table in the FROM clause needs to be surrounded by parentheses:


DECLARE @waypointId INT = 100;   
DECLARE @Sunday INT = 1;
SELECT MIN(nextScheduleDateTime) 
FROM
    SELECT MIN(CAST(CAST(DATEADD(dd, (6 - DATEPART(dw, GETDATE()) + @Sunday) % 7 + 1, GETDATE()) AS DATE) AS DATETIME) + CAST(ScheduleDetail.startTime AS TIME)) AS nextScheduleDateTime



Should be updated like this:

SELECT MIN(nextScheduleDateTime) 
FROM
    (SELECT MIN(CAST(CAST(DATEADD(dd, (6 - DATEPART(dw, GETDATE()) + @Sunday) % 7 + 1, GETDATE()) AS DATE) AS DATETIME) + CAST(ScheduleDetail.startTime AS TIME)) AS nextScheduleDateTime


And then after the sub-queries:

    ) AS Subqueries


Simplifying sub-queries

I believe (but haven't tested, since I don't have your data and expected output) that the WHERE conditions can be updated like below, which would allow combining the sub-queries into one, eliminating the usage of UNION:

DECLARE @waypointId INT = 100;   
DECLARE @Sunday INT = 1;

SELECT MIN(CAST(CAST(DATEADD(dd, (6 - DATEPART(dw, GETDATE()) + @Sunday) % 7 + 1, GETDATE()) AS DATE) AS DATETIME) + CAST(ScheduleDetail.startTime AS TIME)) AS nextScheduleDateTime
FROM WaypointEventLog INNER JOIN
     Schedule ON Schedule.scheduleRef = WaypointEventLog.ScheduleID INNER JOIN
     ScheduleDetail ON Schedule.scheduleRef = ScheduleDetail.scheduleRef
WHERE WaypointID = @waypointId 
AND Completed = 0
AND Schedule.scheduleType = 6
AND 
(( validSunday = 1 AND DATEPART(dw, GETDATE()) &lt;&gt; @Sunday ) OR 
        ( validMonday  = 1 AND  DATEPART(dw, GETDATE()) &lt;&gt; @Sunday + 1) OR 
        ( validTuesday = 1 AND  DATEPART(dw, GETDATE()) &lt;&gt; @Sunday + 2 ) OR 
        ( validWednesday = 1 AND  DATEPART(dw, GETDATE()) &lt;&gt; @Sunday + 3 ) OR 
        ( validThursday = 1 AND  DATEPART(dw, GETDATE()) &lt;&gt; @Sunday + 4 ) OR 
        ( validFriday = 1 AND  DATEPART(dw, GETDATE()) &lt;&gt; @Sunday + 5) OR 
        ( validSaturday = 1 AND  DATEPART(dw, GETDATE()) &lt;&gt; @Sunday + 6 ))


While it only contains the one table scheduleDetail, you can see this illustrated in this SQL fiddle. Feel free to update that fiddle and provide more details about the other tables, plus the data in those tables.

Programmatic way of generating the query above

One approach you could take is to define a CURSOR, and use that to loop through each field, adding the sub-queries to be joined via UNION to a long string. Originally I was thinking a varchar(5000) would be necessary but that was before I discovered the UNIONs could be eliminated.

DECLARE @name VARCHAR(50) -- field name 
DECLARE @rowNum varchar(10) -- row number 
DECLARE @sql VARCHAR(5000) = 'SELECT MIN(CAST(CAST(DATEADD(dd, (6 - DATEPART(dw, GETDATE()) + @Sunday) % 7 + 1, GETDATE()) AS DATE) AS DATETIME) + CAST(ScheduleDetail.startTime AS TIME)) AS nextScheduleDateTime
FROM WaypointEventLog INNER JOIN
     Schedule ON Schedule.scheduleRef = WaypointEventLog.ScheduleID INNER JOIN
     ScheduleDetail ON Schedule.scheduleRef = ScheduleDetail.scheduleRef
WHERE WaypointID = @waypointId 
AND Completed = 0
AND Schedule.scheduleType = 6
AND ('

DECLARE db_cursor CURSOR FOR  -- fetch columns to loop over
SELECT COLUMN_NAME, ROW_NUMBER() OVER(ORDER BY ORDINAL_POSITION ASC) - 1 AS RowNum 
FROM INFORMATION_SCHEMA.columns 
WHERE table_NAME = 'ScheduleDetail' and COLUMN_NAME like 'valid%' and COLUMN_NAME not like '%holiday' 
ORDER BY ORDINAL_POSITION

OPEN db_cursor   
FETCH NEXT FROM db_cursor INTO @name, @rowNum  

WHILE @@FETCH_STATUS = 0   
BEGIN   
    --append WHERE condition
   SET @sql  = @sql + ' ('+ @name + ' = 1 AND DATEPART(dw, GETDATE()) &lt;&gt; @Sunday + ' + @rowNum + ' )'+char(10)+char(13)   -- + CR + LF

   FETCH NEXT FROM db_cursor INTO @name, @rowNum  
   if @@FETCH_STATUS=0 -- all except last condition
    BEGIN
        SET @sql  = @sql + 'OR'+char(10)+char(13)   -- + CR + LF
    END
    ELSE 
    BEGIN -- last condition
        SET @sql  = @sql + ')'+char(10)+char(13)   -- + CR + LF
    END
END   

CLOSE db_cursor   
DEALLOCATE db_cursor

print @sql


That could be used to generate a Table-valued function or View.