We do not have enough data on this route to provide a reliable graph showing how many days before departure it is the best to book.
However, usually the closer it is to the departure, the more expensive the tickets are.
The most popular airlines on the route London Luton - Milan Bergamo Orio al Serio: British Airways (BA).
There is no direct flight on the route honnan London Luton   Milan Bergamo Orio al Serio
The shortest flights with change last about 8h 30min.