This page provides guidance and spreadsheets to help you
determine the Sacred Month according to Plethon’s calendar.
Plethon’s Sacred Month begins on the day following the
conjunction of the sun and moon as determined by astronomy.
Plethon calls the day of conjunction “Old-and-New” (῞Ενη καὶ Νέα,
Henę kai Nea) and the following day (the first of the
month) is called “New Moon” (Νουμηνία, Noumęnia). The
spreadsheets provided here (in two formats) will help you to
determine the dates of Plethon’s ceremonies and rituals. Fill in
the date for Old-and-New immediately preceding the current month
in row 2, and the date for Old-and-New following this month in row
32. (These two cells have colored backgrounds.) Write the dates in
mm/dd/yyyy format. The spreadsheet will then calculate all
the days of the month. In addition, it will indicate whether to
include ”Old Moon” before Old-and-New if the month is full (30
days) or to omit it if it’s hollow (29 days). The spreadsheet also
includes some reminders of the celebrations for the various days.
You might want to upload the spreadsheet to Google Sheets, where
it can be kept up to date and will be accessible on all your
You may find it convenient, at the end of the year, to fill in
the following spreadsheet with the dates for the lunar
conjunctions (Old-and-New days) for the new year. You can look up
the dates on a website such as this, but beware that
Old-and-New (conjunction) is called “new moon” on these sites.
Fill the dates of conjunction into the column labeled
“Old-and-New,” and the spreadsheet will calculate the dates of New
Moon and the lengths of the months (29 or 30 days). Write the
dates in mm/dd/yyyy format. Recall that Plethon’s year
begins with the lunar conjunction (Old-and-New) following the
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