Try this, as a starting point: in cells A1-A12, enter 1/1/2006,
2/1/2006 etc thru 12/1/2006. In B1 enter this formula, then copy and
paste down for the rest.
=DATE(YEAR(A1),MONTH(A1),5-WEEKDAY(A1)+14)

But check the April result: it indicates the first day of the month is
a Saturday, and on the Sunday-to-Saturday calendar that is technically
the first week (even though it's just one day)... so the Wednesday of
the third week, in that scenario, is April 12. If that doesn't do it,
replace this part of the formula above
5-WEEKDAY(A1)+14
....with an IF statement that says if the result is less than 12, or 15,
or some appropriate number, perform a different calculation.