Looking at the code, it doesn't actually change an internal "holiday" object. Probably because we don't actually store such a thing anywhere; everywhere we want to know the holiday, we call HolidayDatabase.getHoliday(), which recalculates it based on KoL + Real World Dates.
Here is what the command calls:
Which looks pretty useless, to me.
public static final void setHoliday( final String holiday )
KoLCharacter.isFullnessIncreased = holiday.contains( "Feast of Boris" ) || holiday.contains( "Drunksgiving" );
KoLCharacter.holidayManaCostReduction = holiday.contains( "Festival of Jarlsberg" ) ? 3 : 0;
KoLmafia.statDay = HolidayDatabase.currentStatDay();
Maybe the command used to do something useful.