It was a Sunday.
September 18th, 1949 fell on a Sunday.
September 18th 1964 was a Friday.
September 18th 1950 was a Monday.
The question was asked on the 18th of December 2011. In 1994 the 18th of December was on a Sunday.
1 September 1994 was a Thursday.
September 3 1994 was a Saturday.
September 19 1994 was a Monday.
September 23, 1994 was a Friday.
Friday
Saturday
September is a whole months, so it couldn't have fallen on any day of the week. The first day of September (in 1994) was on Thursday, and the 30th was on a Friday.
September 18, 1989 fell on a Monday.