The misseri calendar was Iceland's civil and legal reckoning, used to date Althing sessions and legal deadlines, and it operated alongside rather than beneath the Julian calendar the church brought when Iceland formally adopted Christianity around the year 1000: the two systems coexisted through the medieval period, one civil and week counted, one ecclesiastical and month counted, each serving a different institution's needs rather than one displacing the other outright.
The full week counting system did not survive Iceland's eventual adoption of the Gregorian calendar for general civil use, and it is fair to describe the misseri calendar's week by week count as a historical reckoning rather than a functioning one today. Its central division has not vanished from Icelandic life, though. Sumardagurinn fyrsti, the First Day of Summer, marking the start of the summer misseri, remains an official public holiday in Iceland to this day, and Fyrsti vetrardagur, the First Day of Winter, is still culturally recognized as well, so the two-half structure that organized medieval Iceland's entire year survives as a living civil observance even though the week-by-week count beneath it no longer governs anything.