Excel has a function to find the last day of the month. To find the last weekday of the month you can combine a couple of functions. Here is a custom function that also works.

If you are new to custom functions check out this post which has a detailed example.

Avoid the #DIV/0! Error in Excel

The formula to return the last weekday of the month based on a date is something like.

=WORKDAY(EOMONTH(A1,0)+1,-1)

We can create a LAMBDA version like this.

=LAMBDA(d,WORKDAY(EOMONTH(d,0)+1,-1))(A1)

The custom function called fnEOMWEEKDAY is.

=fnEOMWEEKDAY(A1)

The New Name dialog to create the custom function is shown below.

The examples are shown below. The formula in column B is shown in column C.