I think it's a a good idea to contact the delegate several months in advance to let them know you want to organise a competition and you're going to start looking for a venue, and get some idea of availability. It can avoid disappointment if for some reason the delegate is unavailable, and as a delegate it makes it easier planning other comps if you know what's coming so you can space them out properly.
On the other hand, if you've been in contact with a potential venue, and have some idea about costs before contacting the delegate, that's good too. However, you should never confirm the venue or settle on dates until you've discussed it with the delegate.