This depends on the year. July 21st has occurred on every day of the week.
21st July 1970 was a Tuesday.
21st July 1969 was a Monday.
July 21st, 1977 fell on a Thursday.
21st July 2010 is a Wednesday.
July 21st, 2001 fell on a Saturday.
July 21st, 1959 fell on a Tuesday.
Saturday
It was a Thursday.
March 21st 1993 was a Sunday.
July 21st, 1955 was a Thursday.
July 21st, 2005, was a Thursday.