When a date is Entered the Macro will run - excel

Question

How do I add the code that will run automatically when a date is entered. If the code includes other cells - do they need to be populated first. The idea is when today's date is entered and email will be sent. The code Email works but I need for the code to run automatically.
Private Sub Worksheet_SelectionChange(ByVal Target As Range)
If IsDate(Range("A1").Value) Then
'MsgBox "Plase enter a date in A1"
'response = MsgBox(msg, vbYesNo)
'If response = vbYes Then
email
Else
End If
'End If
End Sub
Sub email()
Dim r As Range
Dim cell As Range
Set r = Range("A1:S20")
For Each cell In r
If cell.Value = Date Then
Dim Email_Subject, Email_Send_From, Email_Send_To, _
Email_Cc, Email_Bcc, Email_Body As String
Dim Mail_Object, Mail_Single As Variant
Email_Subject = Cells(cell.Row, "C").Value
Email_Send_From = "Jean#test.com"
Email_Send_To = "Joe#test.com"
Email_Cc = ""
Email_Bcc = ""
Email_Body = "Hi " & Cells(cell.Row, "c").Value _
& vbNewLine & vbNewLine & _
Cells(cell.Row, "D").Value & _
" has been submited"
On Error GoTo debugs
Set Mail_Object = CreateObject("Outlook.Application")
Set Mail_Single = Mail_Object.CreateItem(0)
With Mail_Single
.Subject = Email_Subject
.To = Email_Send_To
.cc = Email_Cc
.BCC = Email_Bcc
.Body = Email_Body
.send
End With
End If
Next
Exit Sub
debugs:
If Err.Description <> "" Then MsgBox Err.Description
End Sub

Answer 1

I believe this is what you want. When the value in A1 is updated, it will first validate if the entry is a date. If entry is not a valid date, you will receive the first MsgBox in code.
If the entry is a valid date, the user will be asked if they want to send the email. If they select No, the macro will end. If they select Yes, your Email Sub will begin to execute.
Private Sub Worksheet_Change(ByVal Target As Range)
Dim Response As String
If Not IsDate(Range("A1").Value) Then
MsgBox "Plase enter a valid date in A1"
Else
Response = MsgBox("Do you want to send email?", vbYesNo)
If Response = vbYes Then Call Email
End If
End Sub
If you do not want the user to have to choose Yes or No to send the email, and you want it to happen as soon as the entry in A1 has been validated as a date, then change your IF statement to this:
If Not IsDate(Range("A1").Value) Then
MsgBox "Plase enter a valid date in A1"
Else
Call Email
End If
Side Note
You should also update this to only execute when a certain range (or maybe individual cell in this case) is changed. Otherwise, the macro will fire when ANY cell is changed. Instead, determine the range that should trigger this macro, and then call the macro when the changed cell overlaps (or Intersects) with your pre-determined range.