In working through each book to better understand the timeline of events in J. K. Rowling's Wizaring World, two facts have become clear to me. One - Rowling has mapped out many of her events quite meticulously, fitting dates and days of the week into a very complete and cohesive calendar. Two - that calendar most often bears little to no relationship to the real world calendar for that year.
As is well understood within the Fandom, the Harry Potter timeline was originally established around one primary date - Nicholas de Mimsy-Porpington 500th Deathday party, which celebrated the anniversary of his death on 31 October 1492. From this date, the events of Harry Potter and the Chamber of Secrets could be deduced to have taken place starting in 1992 and given other peoples' ages, birthdays, and various temporal details, a whole rich timeline could be established.
Much of this work was done by Steve Vander Ark and the other contributors to the Harry Potter Lexicon. Working from evidence given in Rowling's writings and interviews, and working with the real world calendar for the given time period, the HP Lexicon was able to build complete calendars for each of Harry's school years, and timelines for other historical events in the Wizarding World. Some of these Calendars were used by Warner Bros. in developing the films and even appeared as sanctioned extras on the DVDs.
However, trying to fit these events into the real world calendar was often problematic as days of the week given in the text do not line up with the actual day of the week on the calendar. While the Lexicon would often add or remove a day to help correct for these mis-alignments, see Calendar for Philosopher's Stone many times the days of the real world calendar would be used over the days given in canon, leading to oddities such as students taking classes on the weekend (see 14 February from HP Lexicon - Chamber of Secrets Calendar or 16 October from HP Lexicon - Prisoner of Azkaban Calendar).
In reviewing many dates and days of the week as given in Rowling's writing, it becomes clear that although the events are supposed to occur during a given year, her days of the week almost never align with the real world calendar (RWC):
- 1 November 1981: JKR = Tuesday, RWC = Sunday 
- 31 July 1991: JKR = Tuesday, RWC = Wednesday 
- 14 February 1993: JKR = Day of week with Charms, RWC = Sunday 
- 16 October 1993: JKR = Friday, RWC = Saturday 
- 6 June 1994: JKR = Thursday, RWC = Monday 
- 31 August 1994: JKR = Sunday, RWC = Wednesday 
- 30 October 1994: JKR = Friday, RWC = Sunday 
- 24 November 1994: JKR = Tuesday, RWC = Thursday  
- 12 August 1995: JKR = Thursday, RWC = Saturday 
- 2 September 1995: JKR = Monday, RWC = Saturday 
- 14 February 1996: JKR = Sat (or Sun), RWC = Wednesday 
- 18 June 1996: JKR = Thu/Fri, RWC = Tuesday  
Her dates however are most often consistent within themselves for each book, at least over the short run. Setting 16 October 1993 on a Friday, when Lavender Brown learns of the death of her rabbit Binky, leads to a Saturday 31 October, when the third year students first visit Hogsmeade. The arrival of Beauxbatons and Durmstrang on Friday 30 October 1994 results in the 24 November being a Tuesday, which is stated as the day of the First Task of the Triwizard Tournament. This carries over to 24 February, the date of the Second Task, which is implied to be a Wednesday which again is consistent. It even extends to 24 June, the date of the Third Task and the last day of exams, being a Thursday, which is the last day of exams in previous years.
Note however that not all years are quite as consistent, especially when several months at a time are considered. In Harry Potter and the Order of the Phoenix, 12 August 1995, the day of Harry's hearing at the Minstry is a Thursday. 2 September 1995, the first day of class, is set on a Monday. However, these dates are inconsistent with each other and could not both fall on the given days of the week. Moreover, Monday 2 September would lead to 14 February 1996 being on a Friday, whereas it is actually the Hogsmeade weekend setting for Cho Chang's and Harry Potter's first date in the book. Working from other dates, it's clear that Rowling was often working from a calendar, as seen here, but not from the real world calendar and also took some liberties on placing dates.
In Harry Potter and the Half-Blood Prince she finally does align a date and day of the week with the real world calendar. 1 March 1997, Ron's birthday, falls on a Saturday in both the book and the real world calendar. Moreover, 2 September 1996, the first day of classes, is implied to be a Monday in the book and also falls on a Monday is the real world calendar. Whether this consistency over several months of events is evidence of her working from the real world calendar by this time in her writing, or just a fortuitous coincidence, is not clear.
Of course, given Rowling's infamous and oft-cited difficulty with math sometimes, she does have her more grand mis-steps as well, such as Harry Potter and the Goblet of Fire having two Mondays at the beginning of term. It is stated that the Weasley family are preparing on Sunday for the Hogwarts Express the next day, then travel to Hogwarts the next day (Monday), and then start classes the day after which is also Monday.   While the rest of the dates for the book are well structured and consistent, this oddity is hard to explain away, even with magic.
Overall, there is plenty of evidence that Rowling laid out many of her events in a very structured manner. However, in reviewing the specifics of the dates given, it becomes increasingly clear that the calendar from her world most often does not align with the real world, and attempting to use the real world calendar to deduce and assign dates in her world will many times lead to inconsistencies and direct contradictions with the canon she has established.
TL;DR Rowling's dates and days of the weeks are often well structured and are internally consistent, but almost always do not match the real world calendar. Building a timeline for events in the Wizarding World should be based on Rowling's stated canon first and foremost, and not on trying to reconcile or force her events into the real world calendar creating obvious inconsistencies and direct contradictions.
Notes and references
- ↑ HP Lexicon - Philosopher's Stone Calendar
- ↑ Youtube - Chamber of Secrets Timeline - DVD-ROM extra
- ↑ Harry Potter and the Philosopher's Stone, Chapter 1 (The Boy Who Lived)
- ↑ Harry Potter and the Philosopher's Stone, Chapter 3 (The Letters from No One)
- ↑ Harry Potter and the Chamber of Secrets, Chapter 13 (The Very Secret Diary)
- ↑ Harry Potter and the Prisoner of Azkaban, Chapter 13 (Gryffindor versus Ravenclaw)
- ↑ Harry Potter and the Prisoner of Azkaban, Chapter 16 (Professor Trelawney's Prediction)
- ↑ Harry Potter and the Goblet of Fire, Chapter 10 (Mayhem at the Ministry)
- ↑ Harry Potter and the Goblet of Fire, Chapter 15 (Beauxbatons and Durmstrang)
- ↑ Harry Potter and the Goblet of Fire, Chapter 17 (The Four Champions)
- ↑ Harry Potter and the Goblet of Fire, Chapter 19 (The Hungarian Horntail)
- ↑ Harry Potter and the Order of the Phoenix, Chapter 6 (The Noble and Most Ancient House of Black)
- ↑ Harry Potter and the Order of the Phoenix, Chapter 12 (Professor Umbridge)
- ↑ Harry Potter and the Order of the Phoenix, Chapter 25 (The Beetle at Bay)
- ↑ Harry Potter and the Order of the Phoenix, Chapter 31 (O.W.L.s)
- ↑ Pottermore - Explore the Story: Sirius Black
- ↑ Harry Potter and the Prisoner of Azkaban, Chapter 6 (Talons and Tea Leaves) - “Thank you, my dear. Incidentally, that thing you are dreading — it will happen on Friday the sixteenth of October.”
- ↑ Harry Potter and the Prisoner of Azkaban, Chapter 8 (Flight of the Fat Lady) - "“First Hogsmeade weekend,” said Ron, pointing at a notice that had appeared on the battered old bulletin board. “End of October. Hallowe'en.”
- ↑ Harry Potter and the Goblet of Fire, Chapter 15 (Beauxbatons and Durmstrang) - "The delegations from Beauxbatons and Durmstrang will be arriving at 6 o’clock on Friday the 30th of October.
- ↑ Harry Potter and the Goblet of Fire, Chapter 17 (The Four Champions) - "“The first task will take place on November the twenty-fourth, in front of the other students and the panel of judges."
- ↑ Harry Potter and the Goblet of Fire, Chapter 19 (The Hungarian Horntail) - "“Er . . . good luck on Tuesday,” she said. “I really hope you do well.”"
- ↑ Harry Potter and the Goblet of Fire, Chapter 26 (The Second Task) - "With two days left, Harry started to go off food again. The only good thing about breakfast on Monday was the return of the brown owl he had sent to Sirius."
- ↑ Harry Potter and the Goblet of Fire, Chapter 26 (The Second Task) - "“The third and final task will take place at dusk on the twenty-fourth of June,” continued Bagman."
- ↑ Harry Potter and the Goblet of Fire, Chapter 31 (The Third Task) - "Ron and Hermione were supposed to be studying for their exams, which would finish on the day of the third task, but they were putting most of their efforts into helping Harry prepare."
- ↑ Harry Potter and the Order of the Phoenix, Chapter ^ - "Weasley turned to him during dinner on Wednesday evening and said quietly, “I’ve ironed your best clothes for tomorrow morning, Harry, and I want you to wash your hair tonight too."
- ↑ Harry Potter and the Order of the Phoenix, Chapter 12 (Professor Umbridge) - "“Look at today!” groaned Ron. “History of Magic, double Potions, Divination, and double Defence Against the Dark Arts ... “That’s the worst Monday I’ve ever seen.”"
- ↑ Harry Potter and the Half-Blood Prince, Chapter 9 (The Half-Blood Prince) - "“We got so much homework for Runes,” she said anxiously, when Harry and Ron joined her. “A fifteen-inch essay, two translations, and I’ve got to read these by Wednesday!”"
- ↑ Harry Potter and the Goblet of Fire, Chapter 10 (Mayhem at the Ministry) - "“It’s been an absolute uproar,” Percy told them importantly the Sunday evening before they were due to return to Hogwarts. [...] “If the Hogwarts Express crashed tomorrow, and George and I died, how would you feel to know that the last thing we ever heard from you was an unfounded accusation?” "
- ↑ Harry Potter and the Goblet of Fire, Chapter 11 (Aboard the Hogwarts Express)
- ↑ Harry Potter and the Goblet of Fire, Chapter 13 (Mad-Eye Moody) - "The storm had blown itself out by the following morning, though the ceiling in the Great Hall was still gloomy; [...] “Today’s not bad . . . outside all morning,” said Ron, who was running his finger down the Monday column of his schedule." This leads to two sequential days being Monday.
- ↑ H.P Lexicon - J.K.Rowling's Spreadsheet for Order of the Phoenix