Changing the Holidays table and getting "The date specified is invalid" pop-up error is a known problem. Avaya has an MM Service Pack to correct the problem for MM 5.2 and MM 4.0 only. The quick work around option for MM 3.1 was to change the date in the MAS back to 2010, make the Holiday table changes, and then change the date back to current. Not exactly the greatest option.
Chuck
RCT Technologies