European cruises are not small change - you might as well go where you'd really like to!
That said,
DCL lines up its cruise dates very closely with the European school holidays. For example, Barcelona and Rome district school boards will end school anywhere between Jun 21 and Jun 25 in 2019, so you have the most expensive med cruises following these dates. England/Wales schools break for the summer this year from ~Jul 21 to Sep 2, and you will see all the big-ticket Northern European cruises (Iceland, Baltic, Norway) in that timeframe.
These cruises tend to be the most expensive across all of DCL, so I'd avoid them if cost was the main object. We booked last year when they came out, and they have gone up already by ~50% on average.
My suggestions:
1. If you can wait till summer 2020, book early next month when the new itineraries come out.
2. If you want somewhere warm and "cheaper" in 2019, book a late May or early June med cruise now.
3. If you can settle for a cooler but cheaper itinerary, book a September cruise now. Then, start watching. The PIF date for the September cruise will likely be later than the *GT dates for the earlier cruises. One-way itineraries among Rome, Barcelona, Dover, and Copenhagen might go *GT as folks (incl. you, BTW) will have to schedule last-minute flights.