OK

The first part of the formula:

=DATEVALUE(TEXT(A1,"mm/yyyy"))

finds the first date in the same month for any given date (A1).

The second part of the formula:

CHOOSE(WEEKDAY(TEXT(A1,"mm/yyyy")),11,10,9,8,7,13,12)

has 2 parts: the first:

WEEKDAY(TEXT(A1,"mm/yyyy")

determines what is the week day of the first date in the month.This becomes the first argument to the CHOOSE formula.

In CHOOSE you are choosing a value corresponding to the value of the first argument of the formula. (e.g. 1 -first, 2 -second and so forth). So to the first date in the month the formula adds the corresponding number of days to get to the "second Thursday".

So if the first date of the month happend to be Sunday -then you add 11 days to get to the second Thursday.

Monday +10

Tuesday+9

...

Friday+13

Hope this make sense to you.

If you understand that - it will be easy to you to calculate the date intervals needed to get 2nd Monday or 3rd Thursday.

HTH

Eli