Fun with Org Mode Agenda Views

fun-with-org-mode-agenda-views.org

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+#+TITLE: Fun with Org Mode Agenda Views
+#+DATE: 2022-12-28
+I use [[https://orgmode.org/][Org Mode]] extensively, including [[https://orgmode.org/manual/Agenda-Views.html][Agenda Views]].
+I have a =anniversaries.org= file for, well, anniversaries. I have
+added it to the ~org-agenda-files~ list so that entries in there show
+up in agenda views.
+It contains some non-standard entries that some people might get a
+giggle out of.
+* Discordian Holidays
+#+begin_example
+%%(org-anniversary -1166 01 05) Today is Setting Orange, the 5th day of Chaos in the YOLD %d. Celebrate Mungday.
+%%(org-anniversary -1166 02 19) Today is Setting Orange, the 50th day of Chaos in the YOLD %d. Celebrate Chaoflux.
+%%(org-anniversary -1166 03 19) Today is Pungenday, the 5th day of Discord in the YOLD %d. Celebrate Mojoday.
+%%(org-anniversary -1166 05 03) Today is Pungenday, the 50th day of Discord in the YOLD %d. Celebrate Discoflux.
+%%(org-anniversary -1166 05 31) Today is Sweetmorn, the 5th day of Confusion in the YOLD %d. Celebrate Syaday.
+%%(org-anniversary -1166 07 15) Today is Sweetmorn, the 50th day of Confusion in the YOLD %d. Celebrate Confuflux.
+%%(org-anniversary -1166 08 12) Today is Prickle-Prickle, the 5th day of Bureaucracy in the YOLD %d. Celebrate Zaraday.
+%%(org-anniversary -1166 09 26) Today is Prickle-Prickle, the 50th day of Bureaucracy in the YOLD %d. Celebrate Bureflux.
+%%(org-anniversary -1166 10 24) Today is Boomtime, the 5th day of The Aftermath in the YOLD %d. Celebrate Maladay.
+%%(org-anniversary -1166 12 08) Today is Boomtime, the 50th day of The Aftermath in the YOLD %d. Celebrate Afflux.
+#+end_example
+* Discworld
+#+begin_example
+%%(org-anniversary 1957 5 24) The Glorious People's Republic of Treacle Mine Road: "Truth, Justice, Freedom, Reasonably-Priced Love and a Hard-Boiled Egg!"
+#+end_example
+* Shitposting
+#+begin_example
+%%(org-anniversary 0 12 23) Christmas Adam. Comes before Christmas Eve and is generally unsatisfying.
+%%(= 0 (calendar-days-from-easter)) Celebrate the rising of an undead horror from his tomb...
+#+end_example
+"Christmas Adam" came from a [[https://theguilteaparty.tumblr.com/post/69069034510][tumblr thread]] years ago.
+
+"Celebrating the rising of an undead horror" came from a conversation
+between [[https://thecooltable.wtf/@ncweaver/109574943089507499][Charlie Stross and Nicholas Weaver on the Fediverse]].
+
+For the later we need a helper function to calculate the [[https://en.wikipedia.org/wiki/Date_of_Easter][Date of
+Easter]].
+* Calculate Easter
+This is from the Org Mode FAQ: [[https://orgmode.org/worg/org-faq.html#org3d76864][17.8. How do I schedule events relative
+to Easter Sunday?]] Which points at [[https://github.com/soren/elisp/blob/master/da-kalender.el][da-kalender.el]]
+#+begin_src emacs-lisp
+  ;; from https://github.com/soren/elisp/blob/master/da-kalender.el
+  ;; Copyright (c) 2018 Søren Lund <soren@lund.org>
+  ;; This program is free software; you can redistribute it and/or modify
+  ;; it under the terms of the GNU General Public License as published by
+  ;; the Free Software Foundation; either version 3 of the License, or
+  ;; (at your option) any later version.
+  (defun calendar-easter (year)
+    "Calculate the date for Easter Sunday in YEAR."
+    (let* ((century (1+ (/ year 100)))
+           (shifted-epact (% (+ 14 (* 11 (% year 19))
+                                (- (/ (* 3 century) 4))
+                                (/ (+ 5 (* 8 century)) 25)
+                                (* 30 century))
+                             30))
+           (adjusted-epact (if (or (= shifted-epact 0)
+                                   (and (= shifted-epact 1)
+                                        (< 10 (% year 19))))
+                               (1+ shifted-epact)
+                             shifted-epact))
+           (paschal-moon (- (calendar-absolute-from-gregorian
+                             (list 4 19 year))
+                            adjusted-epact)))
+      (calendar-dayname-on-or-before 0 (+ paschal-moon 7))))
+  (defun calendar-easter-gregorian (year)
+    (calendar-gregorian-from-absolute (da-easter year)))
+  (defun calendar-days-from-easter ()
+    "When used in a diary sexp, this function will calculate how many days
+  are between the current date (DATE) and Easter Sunday."
+    (- (calendar-absolute-from-gregorian date)
+       (calendar-easter (calendar-extract-year date))))
+#+end_src
+We can then schedule things relative to Easter:
++ Good Friday :: ~<%%(= -2 (calendar-days-from-easter))>~
++ Easter Sunday :: ~<%%(= 0 (calendar-days-from-easter))>~
++ Easter Monday :: ~<%%(= 1 (calendar-days-from-easter))>~