This may not be the best overall solution, but if I were you, I'd probably do this using a data source, rather than trying to do it entirely in forms.  You'd probably want to create one data table that has the holidays you want to exclude (Thanksgiving, Christmas, Independence Day, etc.) - it probably wouldn't take too long to create that out for several years into the future.  Then make a database view that calculates dates that are in your desired periods (2nd or 4th Thursday) and exclude dates that are in the holiday table.  You could also limit your view to a certain time period (say between tomorrow and six months out).  Finally, instead of using a date picker field in your form, use a drop-down field, populated by the contents of your view.

I haven't tinkered with this at all, it's just a thought...