Matt,

The way the formula works is: First we find the last day of the month with the EOMONTH() function. For the last day of the current month, we use "0" as the month offset. For the previous month, it would be -1. (Either "=EOMONTH(A2,0)" or "=EOMONTH(A2,-1)".)

Then, in the "=CEILING()" function, we subtract the number of days required to bring us back to Sunday from the day for which we are searching. (For Tuesday, it would be -2, so we make the formula so far: "=CEILING(EOMONTH(A2,0)-2...."

We use the "significance" value of "7" because weeks are seven days long (of course).This finds the date of either the previous Saturday, or the following Saturday (Excel dates started with 1/1/1900, which was a Sunday. So if you enter a "1" in cell A2 with the Long Date format, the cell would read: "Sunday, January 1, 1900".). For that reason, the =CEILING() function, using "7" as the "significance" argument returns Saturday, which is always a multiple of 7 in Excel. Now the formula is "=CEILING(EOMONTH(A2,0)-2,7...".

Now we need to return the date to a Tuesday, so we either subtract 4 for the last Tuesday of the month, or add 3 for the first Tuesday of the next month. So the final formula is, respectively, either "=CEILING(EOMONTH(A2,0)-2,7)-4" or "=CEILING(EOMONTH(A2,0)-2,7)+3".

We can work through the math by assuming the last day of the month is either a Monday (assume its value is 9 - for ease of illustration, so the values do not go negative) or a Wednesday (assume value 11). "EOMONTH()" will return either of those days (9 or 11). Subtracting 2 results in the previous Saturday (value 7) or Monday (value 9), respectively. The CEILING() function then returns, respectively, the previous Saturday (value 7, because the ceiling of 7 with significance 7 is 7) or the current Saturday (value 14, because the ceiling of 9, significance 7, is 14). Subtracting 4 results in the previous Tuesday (value 3) or the current week's Tuesday (value 10), respectively. For the first Tuesday of the following month, adding 3 to the Saturday value (7 or 14) returns either Tuesday value 10, or Tuesday value 17, respectively.


Hopefully that helps you understand so that you can modify the formula as necessary. You can even change it to find the second Tuesday, or second to last Tuesday, by adding or subtracting an additional 7, respectively.