Rather than decades the calendar is sometimes broken up into septets. Given the one odd day at the end of the year "day outside time", it means the moon shifts back a day in the week every year.
E.G. year 1 has a lunar event at the end of the week, year 2 has it on Tearday, 3 on Cloday, etc.
There's a lunar shift every 7 years, and a full-shift every 28. Some think this is important, and a few families even plan around it for each consecutive generation (which makes them rather old parents).