Noun: The magical eighth day of straight work without a day off, traditionally landing on the Monday of the second week worked.
Addendum: In most jobs, pay is given weekly or bi-weekly and overtime and double time are incurred on weekends under specific criteria. When it is Fafarnin, pay is always reverted back to normal rates even though there is no break inbetween the weekend and the next week.
1. Holy shit, today is Fafarnin. I need a day off!
2. Happy Fafarnin!