When is Thanksgiving and Why?

get-thanksgiving.js

// Problem Statement:
// Find out how to calculate Thanksgiving in a given year
// Credits: https://coffeescript-cookbook.github.io/chapters/dates_and_times/date-of-thanksgiving
// SOLUTION:

const isDateInPast = (_date) => {
  const now = new Date()
  const nowInt = Date.parse(now)
  const dateInt = Date.parse(_date)
  return nowInt - dateInt > 0
}

const getThanksgiving = (year, nextYear) => {

  const getThanksgivingForYear = (y) => {
    const firstDayOfNovember = new Date(`11/1/${y}`)
    const dayOfWeekNovemberFirst = firstDayOfNovember.getDay()
    const thanksgivingDay = 22 + ((11 - dayOfWeekNovemberFirst) % 7)
    return new Date(`11/${thanksgivingDay}/${y}`)
  }

  const currentYearThanksGiving = getThanksgivingForYear(year)
  const nextYearThanksGiving = getThanksgivingForYear(nextYear)

  const thanksgiving = isDateInPast(currentYearThanksGiving) 
  ? nextYearThanksGiving
  : currentYearThanksGiving

  return thanksgiving
}

let t = getThanksgiving(2021, 2022)
console.log(t)


// My question:
// Why is 11 the magic number?

// Sunday =     0 (4 days till Thursday)  (---) 11 - 0 == 11  (---) 11 % 7 == 4
// Monday =     1 (3 days till Thursday)  (---) 11 - 1 == 10  (---) 10 % 7 == 3
// Tuesday =    2 (2 days till Thursday)  (---) 11 - 2 == 9   (---) 9 % 7 == 2
// Wednesday =  3 (1 days till Thursday)  (---) 11 - 3 == 8   (---) 8 % 7 == 1
// Thursday =   4 (0 days till Thursday)  (---) 11 - 4 == 7   (---) 7 % 7 == 0
// Friday =     5 (6 days till Thursday)  (---) 11 - 5 == 6   (---) 6 % 7 == 6 
// Saturday =   6 (5 days till Thursday)  (---) 11 - 6 == 5   (---) 5 % 7 == 5

// Of Note: 
// Thursday = 4 (0 days till Thursday) (---) 11 - 4 == 7 
// AND (drum roll...) 7 % 7 == 0!
// 11 is the number for which the "day" of the week mods perfectly into 7 (i.e. leaves no remainder)
// For this reason the earliest day of month possible is November 22nd (November started on Thursday)
// and the last date possible is November 28th (November started on Friday)

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