Concierge guests and
Castaway Club members get to book up to 120 days in advance. First time cruisers who aren't staying Concierge can book 75 days ahead. So yes, it's likely that they'll all be booked, especially given the time of year that you're cruising. However,
DCL holds back a few cabanas for onboard booking (assuming they're not being used for a wedding or other special event), so you might have a chance at one of those. Also, cancellations do occur, so you might be able to snag one before or even during the cruise.
If you really have your heart set on a cabana, I'd recommend booking a Concierge stateroom and then being online at midnight of the 120-day mark to make sure you snag one.