Microsoft Excel - WEEKDAY function

I made an interesting discovery about the  WEEKDAY function over the weekend.

What does Excel's WEEKDAY function do?

The WEEKDAY function converts a date in Excel into a number between 1-7 which represents the day of week. 

How do I write a formula using the WEEKDAY function?

=WEEKDAY(serial_number, [return_type])

serial_number - This is the date that you want to convert

return_type - This determines the logic applied to the date. See the table below:

What I discovered

As I was working through the options in the return_type argument I spotted that two of the options are duplicated: 1 & 17 and 2 & 11 are the same.

That made me pause for a moment, its not something I have seen before in a function in Excel (at least I don't think I have).

Let's take a look at a walkthough

Now, it turns out that the reason for this is backward compatibility when Microsoft expanded the options from the original set of three (which they had to retain) to a further seven back in 2010.