To create a date from constituent integers, use MAKEDATE.
With: Y,M,D as 2020,8,25
the date is created as MAKEDATE([Y],[M],[D])
The string day is not sufficient for the date calculation because there are several e,g, Mondays each month. You need the day of the month.
That said, if you do want to convert the string day to an integer, it can be done using CASE, as follows, where day is the string day:
CASE [Day] WHEN 'Sunday' THEN INT(1) WHEN 'Monday' THEN INT(2) WHEN 'Tuesday' THEN INT(3) WHEN 'Wednesday' THEN INT(4) WHEN 'Thursday' THEN INT(5) WHEN 'Friday' THEN INT(6) WHEN 'Saturday' THEN INT(7) END