BankHolidays.scala

import java.time.temporal.TemporalAdjusters
import java.time.{DayOfWeek, LocalDate, YearMonth}
import scala.collection.mutable.ListBuffer
import enumeratum.values.{IntEnum, IntEnumEntry}

case class Holiday(name: String, date: LocalDate) {
  val observed: LocalDate =
    date.getDayOfWeek match {
      case DayOfWeek.SATURDAY => date.minusDays(1)
      case DayOfWeek.SUNDAY   => date.plusDays(1)
      case _                  => date
    }
}

object BankHolidays {

  sealed abstract class Occurrence(val value: Int) extends IntEnumEntry
  object Occurrence extends IntEnum[Occurrence] {
    case object First extends Occurrence(1)
    case object Second extends Occurrence(2)
    case object Third extends Occurrence(3)
    case object Fourth extends Occurrence(4)
    case object Last extends Occurrence(5)
    val values = findValues
  }

  def all(year: Int): List[Holiday] = {
    val thirdMondayInJanuary = findDay(year, Month.JANUARY, DayOfWeek.MONDAY, Occurrence.Third)
    val thirdMondayInFebruary = findDay(year, Month.FEBRUARY, DayOfWeek.MONDAY, Occurrence.Third)
    val lastMondayInMay = findDay(year, Month.MAY, DayOfWeek.MONDAY, Occurrence.Last)
    val firstMondayInSeptember = findDay(year, Month.SEPTEMBER, DayOfWeek.MONDAY, Occurrence.First)
    val secondMondayInOctober = findDay(year, Month.OCTOBER, DayOfWeek.MONDAY, Occurrence.Second)
    val fourthThursdayInNovember = findDay(year, Month.NOVEMBER, DayOfWeek.THURSDAY, Occurrence.Fourth)

    val holidays = new ListBuffer[Holiday]()
      .addOne(Holiday("New Year's Day", LocalDate.of(year, Month.JANUARY, 1)))
      .addOne(Holiday("Martin Luther King, Jr. Day", thirdMondayInJanuary))
      .addOne(Holiday("Washington's Birthday (Presidents Day)", thirdMondayInFebruary))
      .addOne(Holiday("Memorial Day", lastMondayInMay))
    
    if (year >= 2021)
      holidays
        .addOne(Holiday("Juneteenth", LocalDate.of(year, Month.JUNE, 19)))
    
    holidays
      .addOne(Holiday("Independence Day", LocalDate.of(year, Month.JULY, 4)))
      .addOne(Holiday("Labor Day", firstMondayInSeptember))
      .addOne(Holiday("Columbus Day", secondMondayInOctober))
      .addOne(Holiday("Veterans Day", LocalDate.of(year, Month.NOVEMBER, 11)))
      .addOne(Holiday("Thanksgiving Day", fourthThursdayInNovember))
      .addOne(Holiday("Christmas Day", LocalDate.of(year, Month.DECEMBER, 25)))
    
    holidays.sortBy(_.date).result()
  }

  private def findDay(year: Int, month: java.time.Month, day: DayOfWeek, occurrence: Occurrence) = {
    val ym = YearMonth.of(year, month)
    val date = ym.atDay(1)
    date.`with`(TemporalAdjusters.dayOfWeekInMonth(occurrence.value, day))
  }
}