Explanation in German of Feast Days: daskirchenjahr.de
FamilySearch research wiki: https://www.familysearch.org/en/wiki/Germany_Feast_Day_Calendars/
Blog about calendars: https://go.germangenealogycollective.com/post/the-date-is-right-but-which-calendar-was-it-written-in
Church calendars: https://kirchenkalender.com/ (Use this site to determine the date in a record).
Jewish calendar conversion: https://stevemorse.org/jcal/jcal.html
Julian calendar into Gregorian calendar conversion: https://stevemorse.org/jcal/julian.html
French Republican calendar conversion: https://www.napoleon.org/en/history-of-the-two-empires/the-republican-calendar/