Holiday schedule for holidays that change dates

Currently the holiday schedule relies on a statically set day and month which can cause inaccuracies year to year. It would be helpful to have an additional option (for US holidays and likely other countries) to specify holidays in the following format -
First/2nd/3rd/4th/Last “specific day of the week” in “specific month”.

Examples for US holidays include Memorial Day always occurs on the last Monday in May. Another example is Labor Day always occurs on the first Monday in September. If this notation could be used and the exact date could be determined by the back end, the holiday schedule would be a “set up once” item.