Explanation
First, this formula determines the first day of the next month *after* a given date. It does this my using EOMONTH to get the last day of the month, then adding one day:
=EOMONTH(B5,0)+1
Next, the formula calculates the number of days required to "roll back" to the last requested weekday in the month prior (i.e. the month of the original date):
WEEKDAY(EOMONTH(B5,0)+1-C5)
Inside WEEKDAY, EOMONTH is again used to get the first day of the next month. From this date, the value for day of week is subtracted, and the result is fed into WEEKDAY, which returns the number of days to roll back.
Last, the roll back days are subtracted from the first day of the next month, which yields the final result.
Other weekdays
In the general form of the formula at the top of the page, day of week is abbreviated "dow". This is a number between 1 (Sunday) and 7 (Saturday) which can be changed to get a different day of week. For example, to get the last Thursday of a month, set dow to 5.
Note: I ran into this formula in an answer on the MrExcel forum by Barry Houdini.
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Related functions
EOMONTH Function
The Excel EOMONTH function returns the last day of the month, n months in the past or future. You can use EOMONTH to calculate expiration dates, due dates, and other dates that need to land on the last day of a month. Use a positive value for months to move forward in time, and a negative number...
WEEKDAY Function
The Excel WEEKDAY function takes a date and returns a number between 1-7 representing the day of week. By default, WEEKDAY returns 1 for Sunday and 7 for Saturday, but this is configurable. You can use the WEEKDAY function inside other formulas to check the day of week.
