[Xymon] holidays.cfg and "Black Friday"

We have the day after [US] Thanksgiving (aka Black Friday) off. I'm trying to add the day after to the holidays.cfg file but have a problem. Thanksgiving is always the 4th Thursday of the month. The day after varies depending on first day of the month. It's 5th Friday this year but was the 4th Friday last and next years.

It's defined as "Day after Thanksgiving:type=+fri month=11 day=23" which works for this year. I haven't tried testing this for last or next years as I don't know how to.

Is there a better way to do this?

Also, could you please point me in the right direction to testing this.

Thanks, --Robert