After testing it on & off this morning, I did notice something that I didn't expect to happen - if I end up triggering the macro without having the Call Placement file open (not that that would happen much but running a separate macro ended up triggering it). Once it tries to trigger without the Call Placement file, if I try to purposely trigger it, it just sits there doing nothing. It will only work if I re-open both files. Not a huge hassle but definitely not what I expected.
I think my better option would be to just make it a regular macro that I trigger with a button - I already have 3 others so adding a 4th button wouldn't be the end of the world for me.
Would you know how to switch it to a regular macro? I have the following but the line I put in bold comes back with a runtime 424 error.

Sub TEST()
  Dim wb As Workbook
  Dim YEE As Workbook
  Dim ws As Worksheet
  Dim uCols As Integer
  Dim sMnth As Date
  Dim dRng As Range
  
  Application.EnableEvents = False
  
  For Each wb In Workbooks
    If wb.Name Like "*YEE*call placement*" Then
      Set YEE = wb
      Exit For
    End If
  Next wb
    
  'If Target.Address = "$B$4" Then
    For Each ws In YEE.Worksheets
      If ws.Name Like "YEE*MU*" Then
        uCols = ws.UsedRange.Columns.Count
        sMnth = ws.Range("G1").Value
        Set dRng = ws.Range("F:F").Find(Target.Value, , , xlWhole)
        Range("B10:Y10").ClearContents
        ws.Range(ws.Cells(dRng.Row, dRng.Column + 1), ws.Cells(dRng.Row, uCols - 1)).Copy
        Rows(9).Find(sMnth, , , xlWhole).Offset(1).PasteSpecial xlValues
        Application.CutCopyMode = False
        Exit For
      End If
    Next ws
  'End If
  
ThisWorkbook.Sheets("YEE").Range("B4").Select


  Application.EnableEvents = True


End Sub