Source code in the freebasic programming language

' version 23-06-2015
' compile with: fbc -s console

Function wd(m As Integer, d As Integer, y As Integer) As Integer
    ' Zellerish
    ' 0 = Sunday, 1 = Monday, 2 = Tuesday, 3 = Wednesday
    ' 4 = Thursday, 5 = Friday, 6 = Saturday

    If m < 3 Then        ' If m = 1 Or m = 2 Then
        m += 12
        y -= 1
    End If
    Return (y + (y \ 4) - (y \ 100) + (y \ 400) + d + ((153 * m + 8) \ 5)) Mod 7
End Function

' ------=< MAIN >=------
' only months with 31 day can have five weekends
' these months are: January, March, May, July, August, October, December
' in nr: 1, 3, 5, 7, 8, 10, 12
' the 1e day needs to be on a friday (= 5)

Dim As String month_names(1 To 12) => {"January","February","March",_
                               "April","May","June","July","August",_
                        "September","October","November","December"}
Dim As Integer m, yr, total, i, j, yr_without(200)
Dim As String answer

For yr = 1900 To 2100  ' Gregorian calendar
    answer = ""
    For m = 1 To 12 Step 2
        If m = 9 Then m = 8
        If wd(m , 1 , yr) = 5 Then
            answer = answer + month_names(m) + ", "
            total = total + 1
        End If
    Next
    If answer <> "" Then
        Print Using "#### | "; yr;
        Print Left(answer, Len(answer) -2) ' get rid of extra " ,"
    Else
        i = i + 1
        yr_without(i) = yr
    End If
Next

Print
Print "nr of month for 1900 to 2100 that has five weekends";total
Print
Print i;" years don't have months with five weekends"

For j = 1 To i
    Print yr_without(j); " ";
    If j Mod 8 = 0 Then Print
Next
Print

' empty keyboard buffer
While InKey <> "" : Wend
Print : Print "hit any key to end program"
Sleep
End