Hey Adam, show actual code of your sql. Probabbly its Sunday beacuse your sql server Sunday its as default.
You can try somthing like this 
SET DATEFIRST 1;
SELECT 
    DATENAME(WEEKDAY, GETDATE()) AS DayWeek,
    CONVERT(VARCHAR(10), GETDATE(), 23) AS ActualDay
That will change your first day as monday.