See Highlight Whole Day for a previous comment about it.

What is currently happening?

Nothing is happening, I think I have added it in wrong, please see code below:

Public Sub opencalendar()

Dim x As Integer
Dim s1 As String, s2 As String

  Dim today As Date
  today = Date
    Dim todayDay As Integer
    todayDay = Day(Date)
    Dim todayMonth As Integer
    todayMonth = Month(Date)
   Dim todayYear As Integer
   todayYear = Year(Date)
  

    CalculateStartDate
    DoCmd.OPenFOrm "Monthlycalendarf"
    Forms!MonthlyCalendarF!MonthName = Calendar
    
    
    For x = 1 To 42
    
  
        s1 = "Date" & x
        s2 = "Day" & x
        
        
       If Month(Forms!MonthlyCalendarF!(s1)) <> Month(Calendar) Then
        Forms!MonthlyCalendarF!(s2).BackColor = &HD8D8D8
        Forms!MonthlyCalendarF!(s1).BackColor = &HD8D8D8
        Else
    Forms!MonthlyCalendarF(s2).BackColor = &HFFFFFF
    End If
    Next x
        
        
        
         If (x = todayDay And Month(Forms!MonthlyCalendarF(s1)) = todayMonth And Year(Forms!MonthlyCalendarF(s1)) = todayYear) Then
            Forms!MonthlyCalendarF(s2).BackColor = vbRed
        Else
            Forms!MonthlyCalendarF(s2).BackColor = &HFFFFFF
        End If
        
        
    
        
End Sub

Alex I think the is a mistake in the code in that addendum.

If (X = todayDay should be If (Day(Forms!CalendarF(S1)) = todayDay

The code to highlight the day needs to be inside the loop.
Here is a snippet from my own database and a screenshot of this month's calendar with today's date highlighted.

    For X = 1 To 42
        S1 = "Date" & X
        S2 = "Day" & X
        If Month(Forms!CalendarF(S1)) <> CalendarMonth Then
            Forms!CalendarF!(S1).ForeColor = RGB(150, 150, 150)
            Forms!CalendarF!(S2).BackColor = RGB(225, 225, 225)
        ElseIf (Day(Forms!CalendarF(S1)) = Day(Date) And _
            Month(Forms!CalendarF(S1)) = Month(Date) And _
                Year(Forms!CalendarF(S1)) = Year(Date)) Then
            Forms!CalendarF(S2).BackColor = RGB(235, 219, 231)
        End If
    Next

Great! thanks, mate, all ok now.

Can't remember how old it is, don't think there is even the sample db attached so prob just a copy paste error.
Thanks for fixing it, I'll have to get it ported to the blog engine and then I'll be able to update the page.