For centuries the church was the governing agent in the Scandinavian countries and the priest was in charge of recording any and every event that happened in their parish; births and christenings, marriages, deaths, probates, move-ins, and outs, etc. All of these things were recorded in the church books in accordance with the church calendar AKA the Feast Day Calendar.
The Scandinavian Lutheran churches (and several other Lutheran churches) based almost all of their dates on two major events, namely Christmas and Easter. The different Sundays and holidays were given Latin names that corresponded with a specific event on that date or linked to one of the major events on the calendar. This would make some feast days fixed, like Christmas day or New year's day, while others would be moveable, like Easter or Pentecost.
As the priests were recording these events in the church books they would usually use the Latin date referring to a feast day instead of a date that we would easily recognize today. Later they would start double dating using both Latin feast days and dates common to us until they eventually switched to only using the regular calendar system. When they switched was largely up to each priest so the books will be kept differently in the many parishes. Though, in their masses and services, the Lutheran church still refers to the names of the feast days.
Some of the fixed feast days are:
New Years Day/ Novi Anni (1 January)
Holy Three Kings Day/ Epiphan Domini (6 January)
Feast of the Annunciation/ Annuntiatio Beatæ Mariæ Virginis (25 March, until 1771. After 1771, it became a moveable feast day.)
St. John the Baptist/ Festum Navitatis St. Johnnis Baptistæ (24 June)
Christmas Day/ Festum Navitatis Domini (25 December)
The most important movable feast days are:
Shrovetide/ Quinquagesima or Dominica Esto mihi
Easter/ Dominica Sancta
Trinity Sunday/ Dominica Trinitatis
Advent Sunday/ Adventus
The calculations of the moveable feast days are quite complicated (believe me, I’ve tried!). Fortunately, there are many tools online. Click on the links below to access FamilySearch.org’s online feast day calendars for the different Scandinavian countries.
Feel free to email me at ScandinavianFamilyRS@gmail.com with any questions.
Picture borrowed from https://yama-bato.tumblr.com/post/2385245826