Something similar to f57v.

[Koen G]

To the left of that we have a column of letters A to G known as the ferial letters.  In calendars designed to last for many years, these are a clever way of keeping track of the day of the week.  In any year you would only have to know what letter Sunday was (the dominical letter). If you know that all the Sundays are C that year, then all the Mondays will be D, all the Tuesdays E, and so on.

So this diagram you posted says "Want to know what the Sunday letter is?" And then what you have to do to figure it out.
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