What holiday is "Miss All Sunday" named after?
It's not a holiday. In his answer in Vol#18 SBS, Oda-sensei clearly said that "All Sunday" means "All the Sundays". In some English translations, translators add "of the year" but this isn't mentioned in the original Japanese text.

(My translation of said SBS)

Reader:
Hello! Odatchi! I noticed something amazing! It's about Mr. 2
Bon Kurei. It's why only Mr. 2 Bon Kurei has this codename! Numbers
are for males, but Bon Kurei is for a female which means "the end of
Bon" because he is an okama, right?

Oda:
Yes, that's right. Female agents' names are in order of the happiest events, so first:
No. 0 is All Sunday (All the Sundays)
No. 1 is Double Finger (New Year's Day)
No. 2 is Mr. 2 Bon Kurei (End of Bon)
No. 3 is Golden Week (Golden Week)
No. 4 if Christmas (Christmas)

.....something like that. Everyone seems to be puzzled with Double Finger.


Note: There is an illustration of both index fingers raised showing 1 and 1 (1/1 or January 1).

Only the author, Oda-sensei knows what "All Sunday" means, but many Japanese-speaking fans think that it means "everyday is Sunday", while a few others think "all Sundays of the year".