No, there's no built-in function for that.  The option buttons can't
really be part of an option group, either, because an option group can
only have one value.  So you're stuck loading and unloading records from
the table of days yourself.  It's not unlike the code that is needed to
store multiple selected options from a multiselect list box.

You'll need code in the form's Current event to get the initial values
of all the "day" options, and code in the AfterUpdate event of each
option button.  The following is just air code, but I'm thinking you
need something like this:

'----- start of example "air code" -----
Function UpdateDay()

   ' This is a helper function to add or remove a
   ' record from tblMainDays depending on the
   ' value and name of the option-button control
   ' that is passed as an argument.  We require
   ' that the name of the control end with the
   ' day number, 1 to 7, for which the update
   ' is required.

   Dim strSQL As String

   With Me.ActiveControl

       If .Value = True Then
           strSQL = _
               "INSERT INTO tblMainDays(MainID, DayNo) " & _
               "VALUES(" & Me.MainID & ", " & _
                           Right(l.Name, 1) & ")"
       Else
           strSQL = _
               "DELETE * FROM tblMainDays " & _
               "WHERE MainID = " & Me.MainID & _
                    " AND DayNo = " & Right(.Name, 1)
       End If

   End With

   CurrentDb.Execute strSQL, dbFailOnError

End Sub

Private Sub Form_Current()

   Dim rs As DAO.Recordset
   Dim intDay As Integer

   Set rs = CurrentDb.OpenRecordset( _
           "SELECT  * FROM tblMainDays " & _
           "WHERE MainID=" & Me.MainID)

   ' Clear all day options.
   For intDay = 1 to 7
       Me.Controls("optDay" & intDay) = False
   Next intDay

   ' Set those options for which there are records in rs.
   With rs
       Do Until .EOF
           Me.Controls("optDay" & !DayNo) = True
           .MoveNext
       Loop
       .Close
   End With

   Set rs = Nothing

End Sub
'----- end of example "air code" -----

Then you can set the AfterUpdate event property (on the Event tab of the
property sheet) of each of the 7 options controls named optDay1,
optDay2, ... optDay7 to this function expression:

   =UpdateDay()

I think that should do it.