The third Wednesday falls on the 15th of the month.
This means the first Wednesday was on the 1st of the month because Wednesdays occur in a 7-day cycle.
Now, the month has 31 days.
If the 1st was a Wednesday, the days of the week for the rest of the month follow the same pattern
Following this pattern, the 31st day of the month will fall on a Friday.
Hence, the last day of the month will be a Fifth Friday.