Binaryworld - GetRoshHashanah - Retrieving the date that Rosh Hashanah begins in the ... [ VB.net -> Date-Time ]

' Returns the date that Rosh Hashanah begins for the requested year.
' It is important to note that Rosh Hashanah is based on the Lunar cycle so the
' Holiday actually begins at sunset of the day before the date returned by this
' function.

' Note: Many dates in the Jewish calendar are dependent on the date of Rosh
' Hashanah just as Good Friday, Lent, and other dates are dependent on Easter.

Function GetRoshHashanah(ByVal iYear As Integer) As Date
  Dim dDate As Single, iDate As Integer, iDayOfWeek As Integer
  Dim JY, JtoG, iMonth As Integer

  ' Conversion factors
  Const F1 As Single = 765433 / 492480
  Const F2 As Single = 23269 / 25920
  Const F3 As Single = 765433 / 492480

  JY = (12 * ((iYear Mod 19) + 1)) Mod 19

  ' Correct from Julian date to Gregorian date
  JtoG = iYear \ 100 - iYear \ 400 - 2

  dDate = JtoG + F1 * JY + (iYear Mod 4) / 4 - (313 * CLng(iYear) + 89081) / _
    98496

  ' Get the integer portion of the date
  iDate = CInt(Math.Floor(dDate))
  ' Get the fractional portion of the date
  dDate = dDate - iDate

  If iDate > 30 Then
    iDate = iDate - 30
    iMonth = 10
  ElseIf iDate < 1 Then
    iDate = iDate + 31
    iMonth = 8
  Else
    iMonth = 9
  End If

  iDayOfWeek = (New Date(iYear, iMonth, iDate).DayOfWeek) + 1

  ' Postponement Rules
  ' If day of week is Sunday, Wednesday, or Friday
  If iDayOfWeek = 1 OrElse iDayOfWeek = 4 OrElse iDayOfWeek = 6 Then
    iDate = iDate + 1
    ' If day is Monday
  ElseIf iDayOfWeek = 2 AndAlso dDate > F1 AndAlso JY > 11 Then
    iDate = iDate + 1
    ' If day is Tuesday we may need to add two days
  ElseIf iDayOfWeek = 3 AndAlso dDate >= F2 AndAlso JY > 6 Then
    iDate = iDate + 2
  End If

  If iDate > 30 Then
    iDate = iDate - 30
    iMonth = 10
  ElseIf iDate < 1 Then
    iDate = iDate + 31
    iMonth = 8
  Else
    iMonth = 9
  End If

  Return New Date(iYear, iMonth, iDate)
End Function