It was a Monday. Linux cal command:
[.]$ cal 7 1989
July 1989
Su Mo Tu We Th Fr Sa
1
2 3 4 5 6 7 8
9 10 11 12 13 14 15
16 17 18 19 20 21 22
23 24 25 26 27 28 29
30 31
August 31 1989 was a Thursday.
January 31, 1989 fell on a Tuesday.
May 31, 1989 was a Wednesday.
July 31 1970 was on a Friday.
July 31 1996 was on a Wednesday.
July 31 1967 was a Monday.
July 31 1951 was a Tuesday.
July 31 2010 was a Saturday.
July 31 1959 was a Friday.
July 31, 2002 was a Wednesday.
July 31, 1997 was a Thursday.
July 31 1944 was a Monday.