Business-day aware holiday calendars — mirrors
pandas.tseries.holiday with USFederalHolidayCalendar,
AbstractHolidayCalendar, Holiday, and weekday offset helpers.
USFederalHolidayCalendar provides all 11 US federal holidays with proper observance rules.
Extend AbstractHolidayCalendar with a custom list of Holiday rules and observance functions.
Use MO(n), TH(n) etc. to define holidays that fall on the nth weekday of a month.