Calendar / Date Picker - Only Choose Sunday

Why dont you use a simple Userform?

Click on Cell B3 of the attached spreadsheet.

Private Sub SpinButton1_Change()
Range("A1").Value = SpinButton1.Value

TextBox1.Value = SpinButton1.Value
End Sub

Private Sub UserForm_Activate()
MArray = Array("Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec")
TextBox1.Value = Year(Date)

ComboBox1.List = MArray
t = Month(Date) - 1
ComboBox1.ListIndex = t
Temp = 5

SpinButton1.Value = 2016

MyDate = CDate("01/" & ComboBox1.Value & "/" & TextBox1.Value)
DaysMonth = Day(DateSerial(Year(MyDate), Month(MyDate) + 1, 1) - 1)
Mdate = Weekday(MyDate, vbSunday)

Daycount = 1

EndDate = Application.EoMonth(Weekday("01/" & ComboBox1.Value & "/" & TextBox1.Value, vbSunday), 0)

If Mdate <> 1 Then Daycount = 8 - Mdate
   
DayLoop:

ComboBox2.AddItem Daycount

Daycount = Daycount + 7

If Daycount < DaysMonth Then GoTo DayLoop

ComboBox2.ListIndex = 0

End Sub

Private Sub UserForm_Terminate()
Range("B3").Value = ComboBox2.Value & "/" & ComboBox1.Value & "/" & TextBox1.Value
End Sub