The attached screenshot showswhere the search box is (it opens whenever you hover your pic).

What shall it do when the 27th is on a holiday?

And what if the next two days is past the end of the month?

There you go (I tested it on DAX, Daily TF):

TimeFrame(Daily,UpdateOnClose)
If OpenMonth <> OpenMonth[1] Then
   OM      = OpenMonth
   OD      = OpenDay
   ODW     = OpenDayOfWeek
   OY      = OpenYear
   LastDay = 31
   If OM = 4 or OM = 6 or OM = 9 or OM = 11 Then
      LastDay = 30
   Endif
   If OM = 2 Then
      LastDay = 28
      If OY mod 4 = 0 Then
         If OY mod 100 = 0 Then
            If OY mod 400 = 0 Then
               LastDay = 29
            Endif
         Else
            LastDay = 29
         Endif
      Endif
   Endif
   EOM      = OD      //EOM=End Of Month
   EntryDay = 0       //Day to enter a trade
   ExitDay  = 0       //Day to exit the trade opened on EntryDay
   FOR i = OD + 1 To LastDay
      ODW = ODW + 1
      IF ODW = 7 THEN
         ODW = 0
      ENDIF
      IF ODW <= 5 THEN
         IF (i = 27) AND (i + 2) <= LastDay THEN
            EntryDay = i
         ELSIF (i >= (EntryDay + 2)) AND (ExitDay = 0)  AND (EntryDay <> 0) THEN
            ExitDay  = i
         ENDIF
      ENDIF
   NEXT
ENDIF
//
Timeframe(default)
IF ((OpenDay >= ExitDay) OR (OpenMonth <> OpenMonth[1])) AND LongOnMarket Then
   SELL AT Market
Endif
//
If (OpenDay = EntryDay) AND (ExitDay > 0) AND Not OnMarket Then
   BUY 1 contract at Market
Endif
//graph OpenMonth
//graph ExitDay
//graph EntryDay
//graph LastDay