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 Chicago (IL) - Czech Republic: Turkish Airlines (TK).
There is no direct flight on the route from Chicago (IL) to Czech Republic
Il volo con scalo più breve dura 28h 10min.