Imports System
Public Module MainModule
' Zeller's Congruence implementation
' Returns the day of the week for a given date.
Public Function dayOfWeek(d As Integer, m As Integer, y As Integer) As String
' Zeller's output mapping:
' 0 = Saturday, 1 = Sunday, 2 = Monday, ... 6 = Friday
Dim names() As String = {
"Saturday", "Sunday", "Monday", "Tuesday",
"Wednesday", "Thursday", "Friday"
}
' In Zeller's formula, January and February are counted
' as months 13 and 14 of the previous year.
If m < 3 Then
m += 12 ' Convert Jan → 13, Feb → 14
y -= 1 ' Move to previous year
End If
Dim K As Integer = y Mod 100 ' Year of the century (last two digits)
Dim J As Integer = y \ 100 ' Zero-based century (e.g., 2024 → 20)
' Zeller's formula (clearer step-by-step version):
Dim term1 As Integer = d ' day of month
Dim term2 As Integer = (13 * (m + 1)) \ 5 ' month adjustment
Dim term3 As Integer = K ' year of century
Dim term4 As Integer = K \ 4 ' leap years in century
Dim term5 As Integer = J \ 4 ' leap centuries
Dim term6 As Integer = 5 * J ' century correction
' Combine all terms and take modulo 7
Dim h As Integer = (term1 + term2 + term3 + term4 + term5 + term6) Mod 7
' Return the corresponding weekday name
Return names(h)
End Function
Sub Main()
Dim d As Integer = 30
Dim m As Integer = 5
Dim y As Integer = 2024
Console.WriteLine(dayOfWeek(d, m, y))
End Sub
End Module
'
' run:
'
' Thursday
'
