I never know if there's something I'm missing with this, but I always assumed her name came from the phrase 'in a month of Sundays.' Aside from Sunday being a holy rest day, a 'month of Sundays' is a holiday that does not exist, same as how her partner, 'Mr 0', is a number that 'does not exist.'

Huh. I've never heard of that phrase before. Other attached meanings Google tells me include "a very long time" which points toward jer historical interest and purpose, as well as "a time devoid of joy or amusement" which fits her personality. Wild.

I took it as Nat's meaning, being literally "every weekend, every regular day off, every quiet unackowledged holiday that passes without comment".

As Oda put it, the female code-names are ranked in accordance with what he personally finds to be the most important holidays.

Having a day off every week is truly important.

Huh...for some reason, I always thought it was a Christian holiday of some sort, like All Saint's day, or All Souls Day, but looking it up now...nothing.

You might be thinking of Easter Sunday, and Easter is more important to the Christian faith than Christmas.

Theoretically, you could have an All Saints Day fall on a Sunday. The next one is 2026. However, Oda apparently thinks of holidays based on importance, and he apparently always gets Sunday off or something.

https://www.quora.com/Why-was-Nico-Robin-One-Piece-called-Miss-All-Sunday-That-is-horrible/answer/Shubham-D-Raj

The most common usage that I've seen is the "long time" version-- A month['s worth] of Sundays, aka 30 Sundays, aka 30 weeks.

Its little details like this which make me appreciate this fandom more.