The TimerService caters for week days but not for work days. People that want to run jobs on work days usually work around this by querying a database and then deciding whether to go ahead or not. Here are some ideas on how to implement work days.
An IgnoreDays property that takes a list of dates.
A WorkDays section that exposes a Holidays property that takes a list of dates.
I think that should be OK, since there are iCal files containing only holidays. However, perhaps we could provide the dates in a selection list after upload where users can tick dates from the iCal file they want to ignore?